Liner SLBM is not a mystery anymore - it is indeed a ten-warhead version of the R-29RM Sineva missile. A new book published by the Makeyev Design Bureau (my thanks to Alexander, who located the book and published the relevant excerpts) describes Liner as a R-29RMU2.1 missile that can carry "up to ten low-yield warheads with missile defense penetration aids, or eight low-yield warheads with additional penetration aids, of four medium-yield warheads with penetration aids."
The only difference between Liner and Sineva, which is identified as R-29RMU2, is apparently in the type of warheads that it can carry - unlike Sineva, Liner can be equipped with the new warhead that was developed for Bulava and RS-24 Yars. This new warhead is described as a joint development of the Moscow Institute of Thermal Technology, which apparently worked on the re-entry vehicle, and the VNIITF - the weapon lab in Snezhinsk (Chelyabinsk-70), which was responsible for the nuclear part of the warhead. To install this new warhead on the R-29RM missile, the Makeyev Design Bureau developed a system of adaptors that turn a Sineva into a Liner.
The new warhead was clearly in the works for quite some time (in fact, I was told that it's a design that had been tested before the Soviet Union ended its nuclear test program in 1991). One option that I described a few years ago seems to fit - a warhead that weighs about 90 kg and has a yield of about 100 kt (it would be similar to the U.S. W76). My estimate at the time was that a MIRVed Topol-M/RS-24 could carry a heavier warhead, but there is certainly nothing that would prevent it from having the light one.
Now I guess we should wait for MITT to publish a book that would describe the mysterious new ICBM that was tested a week ago. Makes you miss the good old START days when quite a bit of that information was public.