Created a summoner. Eidolon is going to be a mount. I want to ride him bareback (already have this covered) but i'm also planning on standing on his back as well like so:
http://s6.photobucket.com/user/shyam69/media/Croatia%20travelblog/onhorse.j pg.html
...except with a lance.
What kind of save would I have to make to do this? Acrobatics or Ride? Would the feat Trick Riding cover this, if it's ride? I couldn't see the DC being more than 15...
It would be a ride check, but it's entirely DM fiat.
Given that it provides you no mechanical advantage, personally I would just treat it as fluff and not ask for a check at all.
to stand on a mount, using only your balance, no knees, nothing to hang onto. sure might be DC 15, but in combat? if I was GM id say that anytime you get hit, I'd make you do a DC 20 + the damage or get knocked off. maybe 15 + 2X the damage? you'd be at a serious disadvantage to attack to. anyone fighting with a weapon needs to move their feet around. and the double damage from a lance on a charge would be lost due to you not being in the standard seated braced position normal lance fighting has.
but thats just me.
you could make an arguement with a GM that its purely for fluff and as long as you gained no advantage from it, he might let you not have any disadvantage from it either.