Is it appropriate to ask the recruiter what are projected RO rates for the summer before the internship actually starts? How might I go about doing it?
I think that's a reasonable question to ask, even if you may not get a straight answer. If the recruiter doesn't tell you, it's worth asking (1) full-time engineers and (2) former SWE interns at the company.
Keep in mind that history doesn't always reflect present-day conditions, e.g. during COVID in 2020 summer, most return offers were canceled entirely. Sample phrasing:
I'm curious about historical return offer percentages among interns? Is there a certain number of full-time slots available for interns?