You might be able to use implicit tuplets if the groupings stay consistent throughout a significant section of the piece. Notate the first couple measures explicitly and let the musician figure it out from there (most moderately skilled musicians should be able to handle this). (Optionally, to make it more clear, you may want to add 'simile' or 'sim.' after the last explicitly marked tuple.) Again, this should only be used in long passages (preferably the entire piece) with consistent rhythmic groupings and patterns. Also, if the moderately skilled musician mentioned above can't figure it out, this probably isn't the best solution!
Here are a couple examples:
In Chopin's Fantasie Impromptu only the first sextuplet is notated and
the rest are implied with slur markings, a repeated structural pattern
and the fact that there are simply too many notes squeezed in. Also note that the melody (not shown) is written in non tuplets, making writing this in 12/8 impractical.
In Schubert's Impromptu Op. 90 No. 3 we are not given any tuplet
notation at all. Instead, the groupings are inferred by the fact that
there are 6 notes when there should clearly only be 4.
EDIT: And, of course, Moonlight Sonata, as mentioned in the OP comments. :)