I don't think if it's possible in ETLegacy. But i know it's possible in NoQuarter.
When you join a server loaded with bots, you always have one less slot in the team with lesser players. So you must join that team or else you can't play.
So for example when you join you got 6 allies (3 real players + 3 bots)- 5 axis (2 real players 3 bots) going on. If you You don't have a choice to join Allies.
Maybe that could be a solution for this.