It's an interesting idea but I would much rather use a third channel to operate the DT angle. All those linkages look like a lovely place for slop to take place unless the machining of the bellcranks was meticulous.

With two channels you could use a simple set of two control horns. One with the normal throw that is mounted on the second. When the second is activated it would compound with the first for the extreme throw you need. The slots to accomadate the elevator link rods would be pretty weird looking.

This would still need some fancy designing and craftwork but at least it would all be just holes and simple tube bearings.

Also you should be aware that in some rare cases 45 degrees isn't enough to ensure a proper deep stall. The 45 degree angle isn't something magical but rather a guidline. Some models only need 35 to 40 and others need around 50 to get a proper stall and vertical descent. If it isn't enough it can get into a nasty whip stall effect. And there's no way to tell beforehand.

On my free flighters I usually go for 45 and move it a little from there if needed. 45 is USUALLY enough but I've seen some of the more extreme models that needed more. Short coupled old timers generally only need 40 to 45.

And finally.... I've flown in a couple of tighter tree areas that would have been a cadidate site to fly as you are thinking of. Believe me when I say I wouldn't want to fly gliders in such a place. Sure it would survive but it wouldn't be any fun trying to find and stay with thermals.

Obviously this would be electric or gas powered as you wouldn't need this arrangement if you had enough room for a highstart or winch so..... Good Luck if you try this.