That is a good idea to state in rules that you are allowed to remark a stock ball. However, it can be abused as well.
Regarding how long you can play before you must trap up, I think there is a general 30 sec understanding on my local scene.
Personally I see this matter with quite a margin. A trapped ball is not always an advantage. As I pointed out earlier, it will at least cause some distraction.
For it to be a player error that will grant some penalty, the player must clearly have exploited the situation.
Let me put it the other way around. If a player traps up as a cause of his normal multiball play - and do not wait for a TD to free the stuck ball - it is an exploit. Oh, you had a timed Super or playfield multiplier - too bad. I have personally wasted a stacked in Funhouse Super frenzy in competition because I did what is right.
Also, in line with Bowens post. Playing on in a multiball mode with a single ball is to a greater advantage, should be noted by the player and should constitute a penalty if exploited.