Removed engine A/C years ago...Roof air is located in rear third of coach, and is normally adequate while driving, but recent heat wave (98-101) has made it less than desirable in driver's compartment while on the road. I closed off rear discharge vanes on the unit with duct tape, forcing all cooled air forwards. This now gives us a gentle cooled breeze while driving, but I'm wondering - am I setting myself up for any potential problems? (No noticeable change is A/C sounds or other signs of strain)

I doubt it.

Years ago, I had a 28 ft fiver permanently parked on the lower Colorado River, with temps over 120 often. It had only one AC. To get enough cool air up into the bedroom, I ran a 4 inch duct from one front outlet to the BR. The other front outlet was closed, as were the rear ones. It worked fine for the ten years that I had it.

At that time, I only had an analog clamp on ammeter, but it showed no increase in the load, and I never had a thermal shut down.

I would think the only problem to watch for is freeze up of the heat exchanger inside the coach. My Barth would freeze up on the dash A/C inside the dash heat exchanger. I did a lot of engine radiator rearranging on that rig thinking I could up the performance of the dash A/C. I probably made it worse. I now think the real issue was too small a forward heat exchanger.

Good point on the freeze up. I avoided freeze up by using high fan speed when humidity and temps were high. Medium worked pretty well, but low was inviting a freeze up in high temp/humidity.

Thinking back, the slight increase in plenum pressure caused some bleed-through of cooled air into the intake side where it cooled the intake temp sensor, causing short-cycling. A little dum-dum and duct tape took care of that.

Something I picked up from another website. When in high humidity conditions, run the A/C on high fan rather than automatic. This is recommended by some of he manufacturers to keep the A/C ducts from building up condensation and dripping.