Matthieu,
Thanks for the post. This is a very interesting subject that we know and like a lot. In fact we'll be launching a couple of new products and doing some tutorials on the subject very soon.
Answers to your questions as follows:
does this sound like plausible? or am I missing something?
It's perfectly plausible. You've pretty much got it just right. You need a silicone with decent tear strength and the right shore hardness (stiffness) and it needs to be quite thixotropic to stop it from running and pooling at the bottom (to be 'brushable'). The reinforcement that you use also needs to be correctly chosen. In our experience you need a light-weight 'net' that will allow the bag to stretch in all directions (only a little) whilst at the same time keeping it within its elastic limit (so it can't be accidentally torn).
How you seal the bag you've made to your mould can involve a little more planning (if you want to make things really low-labour when in production) but looking at your large and flat mould flange I imagine your plan is simply to clamp it down to the flange using maybe a metal frame and some G-clamps.
ps: I've used peelply on the inside of the car so there is a pattern on the inside that could maybe replace the infusionmesh?!
We think this is possible although we've not perfected it yet. The 'impression' that peel-ply leaves won't be enough 'texture' on the inside of the bag to replace the infusion mesh, not by a long-shot but the idea is right. The best we've come up with so far is putting infusion mesh into the mould and then vacuuming some Elasti-Film over the mesh (you need to use quite a heavy mesh, probably heavier than our standard Medium Flow mesh) and then brushing the bag onto that surface. This seems to leave a good 'impresion' of the infusion mesh on the inside of the bag, although we haven't perfected this just yet.
I hope this helps and look forward to hear how you get on. We'll be adding a brushable silicone bag RTV silicone to our range fairly soon, along with some other interesting innovations that we've come up with (to do with the seal) to our website and tutorials in the next few months.
All the best, Matt