It's not just that they don't have mounting blocks at the track, it's that the horses are NEVER, EVER mounted from a standstill.
When mounting a recently off the track TB, one of the very worst things you can do is have the horse held still while mounting. Most will panic and rear and/or bolt forward if you attempt it.
By your own description, this horse has had previous back and saddle issues - standing still for being mounted will be a challenge for her; and being held/restrained while be mounted will be a big, big challenge.
So my recommendation would be for you to have someone work with this horse for 30 - 60 days just on mounting isssues. I mount from a high mounting block , being old, fat and crippled, but I still allow my horses to move forward while being mounted, which is a hangover from my race horse days.
If you absolutely, positively must have her held for mounting, have someone who has worked with racehorses, or who at the very least, won't stand directly in front on her or "lock up" and freeze when holding her.