I am dubious about checking available disks as part of comitting a proposal.

It is a valid workflow to create and deploy a proposal before all of the nodes that will be part of that proposal have been validated, and (in the Dell builds, at least), we do not have the RAID barclamp configure the disks on the system until after the node has been allocated -- until the RAID barclamp has created the desired RAID config for the freshly-allocated nodes, it is quite likely that they will have zero disks at the time the proposal is being validated.  Addinng a disk-check validation step would force us to fall back to preallocating and preconfiguring all the nodes that will eventually be a part of a swift proposal.