I think there is no way (that I can find) to do this within the rules. Interestingly, there is a paragon path in the EPG that lets you turn your companion into a griffin, but that doesn't help you much.
I wonder, have you considered simply being a beastmaster ranger who has both a lizard companion and a separate mount, or an archer or dual-wielding ranger that has a mount? Both seem like they'd do a decent job capturing the Talenta halfling flavor.
Regarding a house rule solution to let you ride your companion, I'd definitely start by saying that the companion should remain primarily a companion, using the stats and rules for companions. A feat that essentially granted the companion the medium mount property, like the one you describe, seems quite fair, and seems like a simpler and more natural solution than having to pay to purchase the mount/companion.
I could be missing something, but I don't see that you'd really be gaining any great advantage in combat from riding your companion. You'd actually be losing out on its ability to be a flanking buddy and threaten a separate area while mounted. The main advantage would seem to be that you could use the ritual to bring it back when killed.
In fact I would venture that if the feat gave your companion the medium mount property, the clawfoot's Mount power,
and you the equivalent of the Mounted Combat feat for riding the companion only, it would not be unbalanced. It would be a pretty steep cost to make you have to take Mounted Combat as a separate feat, considering that riding the companion doesn't really seem to give you that great of a benefit.
Of course I am not a judge.

And you'd have to make a formal proposal for this house rule if you really want it. I'd definitely think seriously about just taking a regular mount... that way you could indeed have a fasteith!