I would like to raise a discussion about VDR compliance with a wide
range available STBs (Set-Top-Box) for IPTV carrier integration.
The advantage of such compliance is obvious - low cost of STB. The cost
of STB is in the range of 80-150 USD.

I have more or less a proof-of-concept of what you want done. The problem is that is was done as a proof-of-concept and is just that is kludgy at best :-)

1) multicast MPEG-TS streaming support;

This is actually quite simple to implement, and is not that dificult do do in a plugin, that's what I did, I just implemented a Receiver Plugin, that get the TS packets, adds PAT and PMT and send that stream to a multicast address. Everything quite hard coded, but it works, and shows how it could be done.

2) vdradmin support for multiuser STB environment.

That I never did, just some SVDRP commands via .php, and no HTML overlay (I know that it's possible I just did it as proof-of-concept never to release or anything).

Above development could affect the design of VDR in general, I would
like to discuss it in details.

I'm all for it, if anyone want my crude code and improve on it and understand what I mean by kludgy and crude, it's ok to me.