I know I kinda brought FATE into the conversation, and I'm about to continue talking about it, but I promise I'm really really not married to it. I can adjust to any system that isn't Campaign for North Africa-scale super-grognard.
That being said, someone ran some PbP Gundam games using modified FATE rules at Sufficient Velocity ( and ). The second game only came to a halt because the GM, rather than the players, abruptly disappeared, but it otherwise seemed to work alright. It helped that the players generally understood that their characters' aspects shouldn't just be fighting abilities or whatever, and each character was extensively workshopped before approval to match the roleplay's intended tone and feel. It might be worth peeking at it as another case study of using FATE for a mecha PbP roleplay.
In particular, I thought the alterations to the Skills list made it more Gundam-y, especially in the second game.