I don't think you bean-dip.
I think you cut-and-paste.
First, lay the groundwork. Acknowledge all the good things going on, so he feels like you DID actually listen. And then draw some boundaries. So something like: "FIL, I know you want to share this activity with DD. However, you need to remember that DD gets to live her life only once, and that activities she ENJOYS are the most appropriate use of her free time. Pressuring her about this is only going to sour her on it. It's an unfair expectation to put on her. She'll decide when its time, and I hope that if she chooses not to continue, you can keep your understandable disappointment from souring her relationship with you."
Then, you switch to, "FIL, DD and we her parents have NOT forgotten your generous offer. It is not time for DD to make her decision yet. Please stop bringing it up. I cannot allow you to pressure DD about this. And it is completely irrelevant what her parents think, so you please stop pressuring us."
And on the third time, having laid the groundwork for all the reasoning behind it, you switch to your cut-and-paste phrase: "I've asked you to stop pressuring our family about this."
And you never discuss anything else.
"Don't you know she'll get better?" you: "I've asked you to stop pressuring our family about this."
"She'll love it." you: "I've asked you to stop pressuring our family about this."
"We'll pay for it." you: "I've asked you to stop pressuring our family about this."
One key to it: "I've asked you to..." How rude will he feel when you keep pointing out that he's ignoring a request!"
And of course: Be sure you're really listening to your daughter about what she wants. With sports especially, there is just NO sense in doing something because you think you have to, or your mom wants/doesn't want you to. Kids should do things they enjoy.
I think I might point out to my DD that part of the ILs' sport is spending time with them. So if she doesn't like the sport THAT much, she might like the time w/ grandparents enough to build it.
If she likes the sport but doesn't like spending that concentrated time w/ her grandparents, let her know that this is OK. And that her relationship w/ her grandparents isn't static--they won't treat her the same way all the time. And that maybe she can learn the sport and have SHORTER time w/ grandparents. Or learn it, and later if the grandparents treat her differently bcs of her age, etc., she'll have the skills to enjoy it.
Or that she can learn the sport without having to go away for a week with them.
(I have this idea stuck in my head that she didn't enjoy that concentrated time w/ the grandparents somehow. Did I get that from something you wrote about earlier, or was that someone else?)