I'll start by saying that I've never actually tried what I am about to describe. This idea originated from discussions I had with UncleNick but I don't recall if it was his idea or just something I conjured up from the discussions. If anyone tries it and it works, give him the credit. If not, I'll take the blame.
Any way, as most know, UncleNick quite often recommends Dan Newberry's load development approach
This idea for finding optimal seating depth is based on the same process, except substituting OAL in place of charge increments and using fixed charges of Trail Boss.
Using Trail Boss has the affect of eliminating or substantially reducing the effects of barrel timing, because it burns so fast, yet with low muzzle velocity and at relatively low pressure that barrel vibrations are nearly gone before the bullet exits, and muzzle blast, again because it burns so fast that there is very little pressure remaining at the muzzle.
Using this method, you should be able to substantially isolate the variable of OAL to find what your gun likes.
Once that's done, use the standard Newberry method to find Optimal Charge Weight.