I think Dave has a good idea. I have resistance rollers instead of a trainer, but I find that holding steady state at any power level will hurt like the very devil if you do it long enough. Around here, we don't have any climbs over ~500', 6%-10%, but that's been enough for me to be able to ride well in the mountains if I take every one at the max pace I can hold for that duration and keep that up for about 4 hours. If one doesn't nave at least that terrain, then long sweet spot intervals on the trainer, working up to say 2 X 30' X 10', plus the usual VO2max intervals, endurance work, etc.
I've found one-legged pedaling (OLP) on my rollers to be very valuable, 2' OLP per leg, then 2' Z2 legs together, that's 1 set. 1 set at 50-55 RPM, big gear followed by 1 set at 80-85 RPM, small gear, legs together at 90 RPM. Use gears of a size that the last 15" of each OLP interval you are crying for your mommy. Repeat until one can't keep a tight chain going over the top. Work up to 4 of those double sets, about 48' total, not counting warm up and cool down. This works astonishingly well. Once a week. Warning: one might be shocked at how weak one is.
Another good trainer exercise is to hold upper zone 3 in a big gear of 50-55 RPM, working up to 4 X 10' X 5' or 3 X 15' X 5.
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