I'd say as a rising Junior. Here's why:
1. Going as a rising Junior your player will hopefully make contact with coaches at schools he is interested in and discover some schools he has never considered. Easy to walk away with 5-8 quality interactions with schools over the two day period. May get lucky and have some schools your player wasn't considering or had never even heard of APPROACH HIM. Additionally, he may not get a good "vibe" from a HC or AC from a school on his list and may re-think his "top schools" list?
2. Now, as a rising Junior, his list is no longer schools: A, B, C, D, E, and F. But schools G, B, C, M, X, and D. Now you have all Junior year to visit those schools, check out a game later that spring and get a good feel for not only their school but their baseball program. Won't be able to do that if you go as a rising Senior?
I agree, these coaches from the "high academic" schools, especially Ivies, are hyper focused on the rising senior class, mostly. I do know however some very sought after Ivy recruits verbal in early summer of their rising Senior year. If you've got the grades and the skills, that 6th semester report card (aka 2nd semester grades Jr. year) is only a formality. But admissions offices still want to see that you finished Jr year strong.
I'm going to go out on a limb here and say, if you get attention from an academic school in summer of your rising Jr. year, then begin dialogue with the program during your Jr. year, hit the scores they are looking for during Jr. year on SATs and GPA, catch some of their games that spring of your Jr. year, finish Jr. year strong with a 6th Honor Roll report card, I believe some handshakes can be made that June of your rising Senior year. I am not speaking from experience here, just from my "intelligence gathering." This is an avenue I believe is possible.
So, to conclude, if I could send my kid once? I'd err on the side of "more time" by sending him summer of rising Jr. year. I can't imagine any savvy coach won't take the time to "file" a rising prospect to pursue in the fall. In lieu of spending the $ then on HF summer of rising Sr. year you then can spend $250 and go to the camp of the school you are really interested in. Often a committal will be taken at conclusion of the camp on the campus of the school you want to go to.
If you do HF as a rising Senior, it's already mid-August of senior year, with no time to visit new schools you just connected with at the HF event in early August. And no time to visit the school in spring to see the baseball team in action.
If I had to choose? Do it summer of rising Junior year. Because yes, it is expensive.