Glissandi by tuning while performing

First, it must be mentioned that sufficient piano tuning experience is required in order to try this out!

Tuning intervals smaller than a quartertone is fine, at least if the tempo is quite flexible. If detuning a slightly larger interval, it should definately be detuned downwards, and more time is required.
Bear in mind that most string courses have more than one string!
With slightly larger intervals, and if one has to tune many times during the piece, the tuning glissando soon turns into a radical mode of playing that should be avoided on the most expensive instruments, as it may later cause destabilization of the tuning.
Tuning a string back to its original pitch is generally “safer” than the other way around.

On some older instruments, the glissando/tuning will not be smooth, and one will hear noisy cracking sounds.

Examples of works including this technique:

• Martin Rane Bauck: tostados en córdoba en medio de la noche