AFAIK no Canaon DSLR lenses would work.
I believe the current TS-E lenses would work - they have very large image circles to accommodate the movements (although on MF, you'd be limited in TS capability).
90mm TS-E Image circle dia. 58.6mm
45mm TS-E Image circle dia. 58.6mm
24mm TS-E Image circle diameter 67.2mm
17mm TS-E Image circle diameter 67.2mm
The Phase One IQ2 series (60 & 80 Mp) are approx 40mmx53mm The Diagonal is approx 66mm. Hasselblad sensors range from approx 40mmx53mm to a smaller approx 33mmx44mm (Diagonal 55mm). So it all depends on what size MF sensor Canon decides to use.
They could also do as Leica did with their S2 (45 x 30 mm sensor).
Medium format has been anything from 4.5x6cm film to 6x9cm film to today's approx 4.5x6 digital., there is no written in stone definition of Medium Format.