First, correction: just looked at a photo, and, contrary to what I thought I remembered (at least some) PRR SD-9 did have dynamic brakes.
But back to dieseling Madison Hill...
A PRR H6 Consolidation had a tractive effort of 42,170 pounds. With 56" drivers connected by side rods and something like 160,000 pounds on drivers (the only figure I saw, looking at Stauffer's book just now, was 186,000 pounds for the whole locomotive: since the pony truck isn't loaded as heavily as the driving axles, 40,000lbs/axle seems like a reasonable ballpark figure) I'm willing to assume that this was achievable in practice.
An EMD SW-8 switcher weighed 230,000 pounds. (That's the basic model: one with dynamic brakes might have been a bit heavier, but 230,000 is good enough as a ballpark estimate.). EMD advertised it as having a starting tractive effort of 69,000 pounds, but admitted that this was at 30% adhesion. Prudent motive power dispatchers in the earlier decades of dieseldom (= before GE's B36-7 and EMD's 50-series with their advanced wheelslip control systems) thought 18% was a more realistic figure for dispatching: tractive effort you could assume if you didn't want trains stalling on the hill even on rainy autumn (= wet leaves on rail) days. 18% of 230,000 is 41,400 pounds. So: SW-8 ought to be able to replace H-6 on very close to a one-for-one basis. Note that the PRR didn't bother ordering m.u. on its two SD-7: they assumed that there would seldom be enough traffic on the Madison Hill to warrant using both of them. (There was a photo in, I think, "Trains," of a rare, perhaps one-time-only, use of double-headed SD-7 on the Madison Hill when something like a big transformer was shipped by that route.)
So I think they could have made do with a pair of d.b. equipped SW-8.