Please don't take this the wrong way; I'm not trying to be deliberately facetious. But:
Surely the best solution (at least for the future) is to
a) have a whip-round,
b) find a sponsor,
c) get a grant, or
d) scrape together some funds by any other vaguely legal means,
then
a) commision someone to write a new piece to your specifications (I'm sure there would be more than one tMP-er falling over themselves to volunteer), or,
b) better still, sponsor a competition to write new quartet "test-pieces", from which you would have the pick of the crop, at least for a period.
???
I fully realise, of course, that the practicalities are nowhere near as simple as they might sound, but I do think that, as a movement, we could do more to encourage the writing of new music, rather than just sitting around complaining that there isn't enough decent repertoire.
Regards,
Gareth