I can confirm that it was a relatively easy exercise to upgrade the BAe146's altimetry so that it would conform to RVSM requirements.
However, the mods were horrendously expensive and simply not worth the effort.
The reduction in fuel burn between F310 with RVSM and F280 without RVSM was pretty well negligble.
Don't forget that the average stage length was probably less than 1.5 hours so you weren't up there for very long.
From my point of view, the best part of flying the 146 was the fact that it ran out of fuel after 3.5 hours! (Sitting on your ar*e for 14 hours is not the best of fun once the novelty has worn off).