The question of route clearance gets overblown on this forum. Far more importance gets attached to it to underline why this or that will or won't happen. Yes there are restrictions, but from a practical perspective many of these are not insurmountable.
The other thing to bear in mind is that there is a big difference between "not cleared for" and "banned from". For example, Pacers are not cleared for large parts of the network but that does not mean that they are banned or otherwise unsuitable. It's simply that there was never any need to consider their suitability for operation in areas they would never be deployed. This means that they, like many other types of train, could theoretically visit any part of the network. It's simply down to the necessary paperwork having been done.
One of the greatest causes of route restrictions is weight, specifically axle loadings, but, as this thread is about MUs, we can forget that. The other is body profile and protrusions, usually footsteps. Length really doesn't play much of a role, especially when you consider how many places 23m DMUs have been already. As has already been pointed out, they tend to be a tad narrower and taper towards the ends to mitigate against the effect of a slightly larger overhang. It's the footsteps and treadboards that tend to be an MU's undoing. These are usually the reason why restrictions are placed on certain stock in various locations, requiring speed restrictions, isolation of the air suspension or even removal of the steps altogether. But, as I said above, these problems are not insurmountable. The Networker family is the most restricted MU design on the network, and yet a small change to the ride height was sufficient to allow them to be cascaded to the Bristol/Gloucester area without restriction. Consideration will also be given to units being "out of gauge" due to door failures, etc (e.g. with the doors open, a Cl158 becomes significantly wider).
As a side point, EMUs can fail to be cleared for a route due to the potential for interference with signalling. Unless the signalling has been "immunised" then certain classes will not be cleared.
I would also suggest that RETB/ERTMS need not be a barrier to operation. These are radio-based signalling systems that could easily be installed, and I would not be surprised if portable RETB or ERTMS sets are available for use.