So the update:
The Epifanes (heavily stepped on with mineral spirits. Maybe 25% or so? Earballed the ratio. Cut it until it was closer to watery than syrupy.) went on great over the epoxy. I did sand the epoxy to knock down the low spots - since it had to be done in sunlight because my garage is a cluttered train wreck - with 120. Thoughts in no particular order:
-Seems to have adhered well, even in the low spots that didn't get sanded. I wiped several times with acetone, then Frog Tape to get up any lint from the shop rags and any dust that got missed.
-I prefer higher grade masking tape over tack rags for this because tack rags can leave a residue. In fact, I use either Frog Tape or 3M Fineline almost exclusively, not that I varnish or paint that much nowadays.
-My back hurts.
-Really wish I had knocked the epoxy down smoother. I got anxious to get varnish on since it was sitting in the sun and I used the West System I had on hand rather than buying the proper hardener for the application, and didn't want it yellowing. I also want to use the boat at some point soon and wanted to get it done. I can fair down and finish coat any time now that I have the mahogany and epoxy under varnish so it doesn't fade, cloud, and/or yellow. Really should have done it right the first time, but scarcity of spare time got the better of me.
-Even over epoxy, unless you're in a temp controlled garage, you really need to step on Epifanes with mineral spirits. They don't make it any thinner than the last time I used it, it's still syrupy out of the can and would drag pretty bad at can strength. You'll also risk orange peeling and bad fish-eyeing around the inevitable dust specks.
-Varnish was dry but not hard when I put a second build coat on today. Didn't sand between, as you really don't need to on a fresh coat w/ Epifanes. Will know in the morning if it was dry enough. Fingernail soft is okay, dragging soft is not.
-In the sun or on a hot day, keep a jug of mineral spirits nearby. Even though it's low flash, the varnish will thicken fast and occasionally need another splash.
-Keep a can of acetone nearby in case you get any on the fiberglass (I didn't, but I have before, and you need to get it off fast with something much stronger than mineral spirits).
-My back hurts.
-Bugs love this stuff.
-All in all, so far the epoxy base seems to be going about as it's rumored to. Really wish I had knocked it down smoother. The Epifanes itself is still my favorite varnish, and despite some claims to the contrary, is VERY easy to work with and get excellent results from. You just can't slop it straight from the can onto the wood - you need to cut it a bit, but it is very forgiving. You're not going to thin it too much as long as you use decent mineral spirits. No fake tan hue like Cetol.
-Bare spots (inside bottom edge of rails, ran out of epoxy before coating them) sucked up the varnish. Very thirsty spots. Still looks and feels like bare wood after two build coats that were not near as thin as the first coats on bare wood would be (50/50).
-My back hurts.
Now I'm thinking about shaping out about 12" white oak strips to put on top of the rail above the cleats as rub strips. Glutton for punishment. Someday I'll actually re-register it, buy new flares, and put the boat in the water and use it.
