should have been more explict about letting the DM know if there is one. honestly, on guided dives in my experience there is not much 'idle time', as there usually is quite a few people in close proximity. use your best judgment for the situation.
But, I wonder if anyone uses some hand signal that tells their buddy that they'd like to practice stuff. Something hat couldn't be mistaken for anything else.
index finger of one hand pointed to the palm of the other and rotated back and forth; y'know, like its drilling into the palm. then signal whatever drill you're wanting to do. ie, signal drill, signal ok, receive ok from buddy, then tap reg for air share / donate, then signal ok, receive ok, and start drill. this can also be signaled by drawing a large 'S' with your finger in the water, for S-drill as DD mentioned; the drill part is implied by signaling 'S'.
some divers will echo back the statement as well after the initial confirming ok, some will just return an ok signal. there's also the 'you watch me [whatever]' (point at buddy, point at eyes, point at self) sequence, but if you're diving with a permabuddy, that can be a bit verbose if you know how to 'read' each other.
another good one is 'reg switch', which is index and middle finger outstretched like a sideways 'V' and then rotated a half turn back and forth. so, signal drill, signal ok, receive ok, signal swap, signal ok, receive ok, then start process to swap to your backup / octo.
[edit] also, if you've been diving with the same buddy for a long time, there is the 'staged drill' procedure. point at buddy, signal drill, signal ok, receive ok, and then continue on with your dive. then anytime within a pre-dive discussed period of time, say 5 minutes, they can throw whatever drills your way they want without any preamble signal of 'drill'*. when the time period is over, buddy would signal 'cut' (moving index and middle finger open and closed like scissors) then 'drill', and its back to situation normal.
* any drill that would ascend at the end, such as OOA, the 'ascend'/thumb up signal during the process is replaced by 'drill'.