I managed to modify the HU siege mode to give the attackers 60 seconds to capture the base after eliminating the defenders. I've made two versions, one is normal 5 second siege and the other is 10 second siege (kinda like the one from way back if you ever played any of SK's packs). All credits goes to Apex since he made the script.
In standard HU if you kill all the defenders after 10 seconds they automatically "win". This means that they could just kill themselves off if their lives were getting low and often the defenders would never lose. The defenders already usually have a pretty large advantage considering that they are holding a fixed position. Obviously the biggest issue still is that when the defenders do lose it sometimes is from they being spawn killed, but again, on a big map the attackers usually have to run all the way to the other side of the map so it evens out.
With the group I play with we almost always play with two teams on siege and these two modes have made it a lot more fun.
Jack has given me permission to post the files here for anyone to use:
1. I haven't figured out why HX Siege, or for that matter original siege from DS doesn't work on any of the HU maps. This applies to pretty much any game type not included in HU. I don't have 3dsmax so I can't open the maps themselves. Often the issue spawns from the insertion points. If you just load siege from DS into your missions folder and play, two teams will be put on attack and the smoke won't be attributed to any team. Other modes from GR/DS/IT don't seem to work properly either. No clue how to fix unfortunately.
2. Because Apex allowed a server to choose "random" as a gametype in team mode, all the individual HU gametype scripts have calls to pull up random (Apex did some crazy crap to make this work btw and it's pretty impressive). Since I used Apex's script to modify to sure it worked on all maps, I have no idea how "random" will work. Since we never choose random it doesn't really matter for us.