Go carefully. My first, last and every intermediate concern is safety, I cannot urge you this enough. It's why Gromet emphasises that these stories are just stories and nothing more, they're not something to try out in reality. However, if you chose to know differently, on your own head be it. The following thoughts are simply that, thoughts.
Some glassfibre resins create serious heat, so try them on something inanimate first.
Equally, you should consider whether a complete mummification might not pose a serious health risk at a number of levels, which are summarised in the thought that any scenario you are in should allow quick release if something unexpected goes wrong - this is a safety 101 in all forms of play. Play should be exactly that, play and pleasure, and if ever you go somewhere where that is at risk - which is itself sometimes a form of play, after all we see rock climbers and skydivers doing all kinds of daft things for exactly the same edge - then you should consider a quick-release setup. This is why having bandage shears to hand before you start is an essential, the headache is they don't work so well on casts. You may have a reciprocating cast saw to hand, however what happens if you have to apply CPR to someone when their torso's so coated you can't get at them? This is why medics don't go there. The legal ramifications are horrendous too, does the Dom really want to risk Homicide 3 Manslaughter charges, 20 years sharing a cell with Albert the Hulk if somethng goes seriously pear-shaped? They'll be a damned sight less dominant after that, I assure you.
That being said, this form of encasement almost goes too far. What you might consider is whether it might not perhaps be wiser to modularise the cast so that a body moulding comes in two parts, front and back, with locating pins and flanges to take some kind of lock or tie - a plastic one, for example, will hold it completely closed yet allow very rapid release, four quick snips. Similarly for the limbs. The greatest risk there is if something gets pinched putting it on! Design the seams so they slide past each other male half on one side, female on the other. And for the effect, the dom makes it seem the entire suit is to be sealed. The advantage is that you're not stick for hours while the resin sets off - OK, there are quick-set resins, however...
What I'm pointing you towards is a kind of personally-fitted suit of armour made of glass or carbon fibre rather than steel. The Mummy Walks. There are plenty of sites describing that process. If the mummy shouldn't walk, then use mould release gel at the seams between the parts so you can create the suit stepwise, starting from the cuirass before moving on to the pauldrons and arm greaves, using dowels to keep the holes in the flanges located, and similarly down the legs and up to the helm. Instead of leaving gaps as a suit of armour does, take the solid right up to the line of the skin fold at the joint - look at your elbow inner joint to see what I mean, there's a clear line showing where the upper and lower arm meet, that's where each part of the cast goes to. By butting up the pieces like this, you create an utterly rigid armature in controlled stages. A similar concept applies to the head, do the front first, preferably while wearing a latex hood coated in release lube or vaseline: sometimes it's a three-stage process, making a female mould of the real head in something like papier-mache. so nostril tubes allow the patient to remain breathing, then fill that with expanded foam as a male cast (don't forget the release agent, vaseline is your friend), then cover that with more papier-mache to give strength and a slight airspace stand-off to allow the air to get out when the final outer cast is made as a female on that. You don't want that helmet to create a vacuum fit to the skin, the latex and papier mache make a head space just a tad larger than the sub's real head! Similarly, for a female mummy (well, aren't they all?), the question of what to do with all that hair may be relevant. JG Leathers' site shows exactly how he did just such a helmet using a form of expanding metal mesh, Formetal Ysilon - he couldn't hammer it to shape onto his model's head, so he moulded a duplicate head as I describe and fitted it to that.
Just be careful not to breathe in resin dust, when using a cast saw or files, it's potentially a serious health hazard - this is why they sell facemasks for those working in it. You could use latex gasmasks fitted with breathing canisters for both parties - and if you wanted to really load the sub, rebreathing the dom's waste air is just about feasible, we're fairly inefficient in what we breathe out, there's lots of oxygen left, however it is a really stuffy experience much like using a rebreather bag! It might even be wise to put such a bag in as a reservoir halfway down the pipe so the sub and dom are able to breathe at their own rates. Try it as a separate learning experience first.
Similarly, while rigging locking flanges, you might also consider freestanding flanges around the body periphery as you look at it from the front, to secure separate bondage to. They say that suspending such a cast from a surrounding frame using bungy elastics is close to weightlessness.
And finally, this is entirely at your own risk. My first, last and every intermediate concern is safety, I cannot urge you this enough. It's why Gromet emphasises that these stories are stories and not something to try out in reality.