There is a stabilizer function in the settings/controls tab for the Me 109. it does work like trim on a slider. Trim isn't modeled though, you are right. Some, myself included, use the above mentioned control assignment for trim on that plane.
I know this is an alpha, but I'm really disappointed that trim wasn't included yet. It's a basic and important function - especially for the LaGG.
I'm not surprised, as Trim was completely omitted from RoF (I know it's not realistic for MOST of the aircraft from that period, but as an option it would have been nice). I have a feeling it was omitted more because they didn't want to deal with it, with 'historical accuracy' being just a cover. The lack of inclusion in the BoS alpha supports this.
Here's hoping this is a sooner, rather than later, feature. As you mention, it's an Alpha build and all, but this seems like a feature that should be baked in right along side the other axis controls.
A bit annoyed the controls are listed there though, took me 20 min of toying around before i realized it wasn't that I was doing anything wrong.