This is an esoteric one, but I believe that it is appropriate for inclusion here, despite the vast gulf in time between now and thread-creation. Bear with me please...
There is an old 3.5 general feat called
Mighty Steed that was published in association with the Dragonlance setting (it appeared in the 'Knightly Orders of Ansalon' and 'Bestiary of Krynn' publications) that could be taken by a creature intended to serve as a mount, which enabled the creature to be considered as one size category larger for the determination of carrying capacity and the maximum "able-to-ride" size category.
In Pathfinder, the
Undersized Mount feat addresses part of the same concern, and I can see that the v13.14 release of the Pathfinder files for Hero Lab (
http://forums.wolflair.com/showthread.php?t=58263) corrected a bug in the HL implementation of the latter feat.
While the 13.14 bugfix addresses the implementation of the specific feat, I can find no indication of any user-facing mechanism by which the logic limiting the population of the Mounted circumstance drop-down of available mounts can be augmented, modified, overridden, or suppressed.
If I am incorrect in thinking that, at present, the ability for the Undersized Mount feat to modify that size-based filter is hard-coded, I apologize and would appreciate it if someone could point me in the correct direction to script up my own fix. Otherwise, it sure would be spiffy if a user-facing way to change that "valid mount determination" procedure could be provided at some point.
(As an example, I'm imagining the potential for a pair of custom-designed races, or a Summoner/Eidolon archetype, that results in two medium creatures who can operate independently but are much more effective when one is mounted on the other, and while I know that I could just give "Undersized Mount" as a bonus feat to the rider of the duo, it would be more thematic if the ability only worked (and could be configured in HL to reflect this) with the intended partner/mount creature...)