  Emittance exchange beamlines employ transverse masks to create drive and
witness beams of variable longitudinal profile and bunch spacing. Recently,
this approach has been used to create advanced driver profiles and demonstrate
record-breaking plasma wakefield transformer ratios [Roussel, R., et al., Phys.
Rev. Lett. 124, 044802 (2020)], a crucial advancement for efficient witness
acceleration. However, since the transverse masks are individually laser cut
and installed into the UHV beamline, refinement of the beam profiles is not
possible without replacing masks. Instead, this work proposes the use of a UHV
compatible multileaf collimator as a beam mask. Such a device permits real-time
adjustment of the electron distribution, permitting greater refinement in a
manner highly synergistic with machine learning. Beam dynamics simulations have
shown that a practically realizable multileaf collimator can offer resolution
that is functionally equivalent to that offered by laser cut masks.
