I don't think you are out of line at all. And as someone who has been similarly affected in past dealings, I don't blame you in the least for trying to do this.
That said, I will be very interested to hear the responses you receive as you approach vendors with this proposal. I expect you are going to get a lot of blank stares / confused voices / etc, and likely flat out refusals. Not that you should, what you are asking for is completely reasonable. But most small businesses of the type who are in the car hobby aren't likely to have on-staff counsel, or even a relationship with someone who can be counsel for them. And as such, I expect most of them won't even consider signing an agreement like you are proposing.
I hope our hobby hasn't truly gotten to a point where this type of step is actually necessary...
