I've got a set of troy lee designs Moto pants and they have the removable leg. I've never had an issue with them melting or getting hung up on anything and I do alot of off trail tree and hill riding.
And don't knock the shorts I've got a couple sets of Moto shorts from TLD too and that's all I wear in the summer. I don't know that I would race in them but for my riding there awesome. The two times I've burned my leg was completely my fault, picking up a wiped out 250 for a buddy and got the header to the calf and sitting backwards on my ds talking swinging my legs and the back of my leg hit the endcap on the pipe.
2015 freeride 154 aaen pipe, bondi box, diamond S hood
2013 600RS 128x2"
2009 ds450 vin#0001 13:1, dual ex cams, ron woods, pro armor, bcs, lonestar, walsh, blingstar, odi, itp, maxxis, AMR
2008 ds450X Yoshi, pink wire, AMR
2007 renegade 800 30" zilla's, snorkels, skids, high clearance arms and warn pro vantage winch
2006 renegadeX 1000sdi garbage motor got a 809 triple on the shelf and a 151" finger track
2003 7c special edition race ported 827 16:1 crankshop twins, 42mm bored TMx carbs