Hi David,
I have no idea what you mean by "an interface could be added". Grob
interfaces are a fixed component of the grob data structure: you cannot
add or remove them at will. A certain grob type either has an interface
or not. It's not a per-grob decision to make.
Looking at the list
<http://lilypond.org/doc/v2.18/Documentation/internals/graphical-object-interfaces>,
I don’t see a "measure-centred-interface" or something obviously
equivalent. So what I meant was: can/should such an interface be
created [i.e., "added to that list"], and then the appropriate grob
types be made to have that interface as part of their makeup?