I was thinking about this, and I have a proposition.
How about if I make it so that the snowglobes won't award perks if you have 7 or above in a stat, and if you get the perk, it marks you as being unable to get that particular stat implant?
Standard disclaimer: This is your mod, do what you want.

My opinion however... neither sounds like a particularly good idea.
For the "only if your stat isn't 7 or higher"... sure that cuts down the power some, but now you have a system where someone with stats of all 5's and 6's is rewarded more for exploring and finding the snowglobes than someone with 4/7/4/7/7/4/7 stats for example. Last thing you want to do is make someone feel "punished" for playing their character the way they want (ie, "Aww man... I found this new snowglobe for the first time... had I taken a Str of 6 and put one more point to my int, I would have gotten a benefit from it... but not now...") Or worse, someone taking intense training to bump to 7, and then finding the snowglobe...
Limiting the stat implant... first it doesn't really make much sense. Second, it again feels like something that encourages min/maxing too much. Ie, making sure to use what stat implants you can get based on your End, to raise you attribs that are already 7 or higher, using snowglobes to fill in the rest.
Personally I'd rather see something different from the snowglobe perks... even keeping close to the SPECIAL aspect, just by doing something that is close to another perk related to that SPECIAL and/or the benefits of that SPECIAL.
Example:
"Str" snowglobe: Instead of +1 Str, gives you a 20lb carry bonus and a +1 Str to weapon handling checks (so basically a weak version of both the Strong Back and Weapon Handling perks -- while still stacking with the original perks)
"End" snowglobe: Instead of +1 End, gives you +15 hit points and +1 DT (so a weak version of Life Giver and Toughness perks)
"Int" snowglobe: Instead of +1 Int, gives you 1 extra skill point per level and skill magazines last 50% longer (weak version of Educated and Retention perks)
By using weak versions of multiple other perks, you increase the chances that everyone will find every snowglobe at least -somewhat- useful, rather than a total write-off. (Example: a Str10 character won't care about the +1 Str to weapon handling checks, but the 20lb carry weight increase is still useful)
Plenty of other perks to look at for inspiration along those lines: Action Boy, Better Criticals, Quick Draw, Finesse, etc.
This is all just some rough ideas off the top of my head -- but IMO, that is a direction you could look at to avoid the problem mentioned before.