different rhythmic units for tuplet's numerator and denominator

[Uri Sala]

Hi,
The only way one can write a tuplet such as "3 notes in the duration of 7 16ths " in a consistent and correct way with a general law of tuplet notation is to assign the rhythmic value of a quarter note to the 3 notes, so that the total value is of 12 16ths (the first rule of a correct treatment of tuplets is that the tuplet is of more value than what it substitutes.
Is it possible to assign different rhythmic units to the numerator and the denominator of a tuplet? Is it possible to show those rhytmic figures next to each?

Thanks for the help,
Uri