Wanted to put my extra Eotech on my 870 with Mesa Tactical side saddle.  It hangs over the side too far, so I can only load shells from below.
Would an Aimpoint Micro T1 or H1 be a good option?  Any other red dots that are thin enough to allow top loading of the side saddle?

I have the same problem with my Mossberg. One of the instructors in the Magpul shotgun training video uses a Aimpoint T1 and doesn't seem to have any problems. Another option is to load your two shells for breech loading in the rear (assuming your optic is pushed all the was forward on the rail) and your remaining loaded down for tube loading in the forward slots. I don't know if they are easy to find but you can get one of the rails that runs the length of the barrel. Then you could push your optic forward and completely clear of the receiver.

Yep, my T1 Micro worked with my Benelli M4 when I had the mesa side saddle on, they should be similar in width (the 870 and the M4s90). could load shells through the top or bottom.