Just a thought, but what if your xref'd title block had every size you need, in that one xref?
That way, when you are in your layout tab, you'd have 3 title blocks showing (for 3 dif sizes) and you'd just have to create your viewports and what not in the right one. Maybe even use XCLIP to 'hide' the non-relevant sizes?
It would affect your plotting situation, a bit, but could be an easy workaround. You'd just have to insert the xref such that the desired title block had it's base point at 0,0, or just use "Window" to select the plot area, and have defined page setups for each size.
That way you could have all the same information, etc, in each title block, and only one file to insert. May create need for a little adjustment to standard practices, but I don't think it'd require much, and would still be a big increase in efficiency.
You'd probably just have to have an empty area in your title block for client logos and what not, and just insert the proper one into the xref file on a per-job basis. Maybe make it a dynamic block for easy swapping/switching.