Boulderman,
your eveluation is correct. The TCAS in deed does include the performance-factors of the aircraft, and in 42000 feet (assuming that is close to the max. altitude under given conditions -A/C weight, Temp., etc.), the TCAS would not issue a climb-command on RA, but as you correcly stated, to maintain the present altitude, while the other A/C is commanded to descend.
Of course, a Learjet operating at 42.000 feet is likely to receive a climb command on RA, while a 747 will certainly be commanded to either maintain altitude or to descent by the TCAS.
Cheers