First, yes, red plastic foil is still available (used in other printing techniques), though it becomes more difficult to locate. I don't know whether the stuff I use is the same as ruby lith: it does not come in tapes, but in sheets which I cut.
Bob Herbst, who responded in this thread earlier, has written a description of the direct reversal process at
http://unblinkingeye.com/Articles/NbyR/nbyr.html
Maybe you would like to add something, Bob? Liam Lawless's original description seems to be no longer online. I have the pdf here, but I would feel better if interested people contacted Liam himself about it.
About in-camera negs: why not develop a few mf-negs accordingly, and print them in a process you would like to try? (Which is it, by the way?) Or, build a pinhole camera, and use sheet film inside.
About interpositives: they should be rather flat and dense. I don't think you will get the right quality using lith film for an interpos. You may use it for maskin the interpos, however. Personally, I think this is a tedious procedure, and I might consider producing digital negs if this would be the only route.
I currently make enlarged negs of up to 20x24", and through reversal I can do this with reasonable effort, at a reasonable cost (considerably cheaper than digital negs, as fas as I can see), and, if I get it right, at absolutely first-rate quality.
My suggestion to those interested is: read Bob Herbst's article first, then ask specific questions.
I might just add here the following qualification:
a main problem of direct reversal with lith film is highlight contrast, particularly if your original negs are a bit on the contrasty side, as are mine. If you expose too long, shadow values are lost, and if you expose for the shadows, highlights will lack contrast. Two solutions, either single or in combination: a) use the first developer at half strength for the double time. b) sandwich two enlarged negs, one for the shadows, one for the highlights.
Also, I frequently use selen toner to optimize the neg.