The issue remains that every student in a MSTP is considered a MSTP student for reporting purposes to NIH. So it is not just funding (although that is critical). Study sections don't like to have an unfunded 2nd class of students. A few elite programs have done this over the years, and it has backfired on them due to criticism (losing some slots during review; it would have been this or other thing). The new MSTP FOA from NIH specifies that the target for number of T32 slots is about 25% of the number of trainees. They add "Institutional commitment and support for the proposed training program are important elements of the application" and then indicate that full funding of trainees is among the benchmarks.