If the sleepers were to move to Waterloo, then Sheepcote Curve would finally see some regular use again; it looks very rusty when seen from the KPA-WWR parliamentary train!
Given a wide variety of freight frequently uses the WLL, I'd be suprised if a rake of MK3s would turn out to be an issue. If nobody has checked, however, then gauging would need to be done first, just to make sure.
There are currently signs for platform 20 at Waterloo, so presumably some work has been done previously to make the platform usable by normal SWT services? Since there are two trains for the Caledonian sleepers, and they arrive at roughly the same time, then this would possibly mean retiming them, or bringing two platforms back into usage at Waterloo.