I actually tried to come up with a widescreen solution for Jazz Jackrabbit 2 about a year and a half ago and had limited success. I was able to boost its resolution from 640x480 to 720x480 and 848x480 (a weird one, I know), but anything higher than that resulted in an access violation error. I didn't think it was enough of an improvement to warrant posting, but since you asked...well here you go!
80 02 00 00 51 c7 44 24 5c
e0 01
80 02 00 00 0f 85 04 01 00 00 81 f9
e0 01
80 02 00 00 75 2e 81 78 08
e0 01 00 00 75 25 80 78 0c 08 75 1f b8 01 00 00 00 c3 81 f9
80 02
80 02 00 00 75 0c 81 f9
e0 01 00 00 0f 84 be 00 00 00 b8
80 02 00 00 ba
e0 01
80 02 00 00 bd
e0 01 00 00
80 02 00 00 52 c7 44 24 1c
e0 01
80 02 00 00 88 44 24 24 88 44 24 26 8d 44 24 04 c7 44 24 0c
e0 01
Use a hex editor like Be.Hexeditor or XVI32 to replace the 80 02s and e0 01s with your custom resolution's corresponding codes.
[spoiler]
640x480 - 80 02 x E0 01
720x480 - D0 02 x E0 01
848x480 - 50 03 x E0 01
800x600 - 20 03 x 58 02
1024x600 - 00 04 x 58 02
1024x768 - 00 04 x 00 03
1152x864 - 80 04 x 60 03
1280x720 - 00 05 x D0 02
1280x768 - 00 05 x 00 03
1280x800 - 00 05 x 20 03
1280x960 - 00 05 x C0 03
1280x1024 - 00 05 x 00 04
1366x768 - 56 05 x 00 03
1440x480 - A0 05 x E0 01
1440x900 - A0 05 x 84 03
1600x900 - 40 06 x 84 03
1600x1200 - 40 06 x B0 04
1650x1080 - 72 06 x 38 04
1920x1080 - 80 07 x 38 04
1920x1200 - 80 07 x B0 04[/spoiler]
Again, 720x480 and 848x480 were the only two I was ever able to get to work. But hey if you're feeling lucky, by all means try something higher.
Oh and last I checked there was still an active Jazz Jackrabbit community so check them out. It wouldn't surprise me if they've got a fully functioning widescreen fix by now. It also wouldn't surprise me if they weren't forthcoming with it, because as it's a pixel based fix, a higher resolution means you get to see more of the map, which could certainly be considered a cheat in multiplayer games. They'll probably frown upon that.
640x480
720x480
848x480