Button spamming as special infected

Solution 1:

Spamming any buttons has no effect on the damage dealt.

Once you connect with an attack as a special infected, you lose control (for the most part) of what you're doing and continue on mindlessly. This is true for the Charger, Smoker, and Hunter. The Jockey loses the ability to deal damage by clicking (although still does damage over time), but can still steer the survivor around.

If you are knocked off (except as Charger, since you can't be), you stumble back and then can attack normally again.

Solution 2:

No. Repeatedly pressing the attack buttons does not inflict additional damage when you have an incapacitated survivor.

However, it can be beneficial to repeatedly press the secondary attack button to claw a survivor (as any special infected, sans Tank), as it is difficult to know the exact timing between one attack and the next. For example, as a Hunter, if you have a survivor pinned (or grappled, as stated in your question) and you are pushed off them with a melee attack, immediately begin spamming your secondary attack to get off a few scratches before you are killed. It will not increase the damage dealt to the survivor in terms of damage per second, but it will allow you to perform successive attacks as quickly as possible for maximal damage.