I don't think it's possible to enable palm rejection in software, for the Magic Trackpad. I've been trying to find a way to do the same thing, with no luck. I found a feature request for the Better Touch Tool (BTT), where a developer closed the ticket as not being possible — even for such a robust app:
Poor Palm Recognition on the external Magic Trackpad 2 #1448 | BetterTouchTool | Github
However, it may be possible to solve with some physical changes to the keyboard/trackpad tray. I have considered building up a small rim around the top corners and along the sides of the trackpad on the similar keyboard/trackpad tray which I use, The Wedgie (I bought my tray from an Etsy listing by the same "StUDS" merchant, which has since been removed; appears that they still sell it directly from their site, as I've linked to here). I have not yet done so myself, but I think that I may be able to add a ridge of Sugru (same color as my tray), on the tray right next to where the trackpad sits, to prevent my palms from resting on the trackpad.