I edited the last after it being posted for a few hours - anyone following along should re-look at it, I made a couple of errors in the order of things to do, and had a step that wasn't needed. It is now corrected.
The downside to doing this for other flight surfaces is that this is the last thing done before the instructions go out to the servos. The amount of rotation for "up" vs "down" from the servo center is not equal, and there is no way for the Mixer to know this. So this could give inaccurate mixing results in other times and places. Hence why "weight" and "offset" are usually the preferred options. Down the road, this setup might be hard to deal with if you want to get fancy. But initially, it's very easy and gets you in the air. It is something to use when matching dual elevator servos, or the like.
OK, now you want to have the elevator move a little with the application of flaps.
So, I would create a new line on the elevator channel in mixing. Since the flaps are using "MAX" as a source, do the same here. We know it won't take much elevator, so make the weight small - maybe 5? You may have to make it negative to get the elevator to go in the correct direction. Tell it to use SA- for a switch.
Make another line exactly like above, but make the switch whatever it is for full down flaps (either SA↑ or SA↓)
Test fly, and see if it is too much or not enough elevator for the amount of flaps. Increase/decrease the amount of weight on the appropriate line until you get the desired amount for each.
Down the road we might want to discuss how to make "in flight adjusters" for the elevator weights, so you can just dial it in while flying, rather than fly-try-land-adjust and repeat. But that pre-supposes a moderate understanding of the system, so might get a bit deeper than wanted at this point.