Which card(s) are you considering trying to make spend on?
So far I haven't seen any data that (allowed) Plastiq spending doesn't count for sign up bonuses.
Just certain gift card purchases (Simon Mall cards on Amex sign up bonuses recently) from among the current methods. Training your cards to allow high value purchases at some merchants may be another matter. Plastiq seems to work just fine, as it should considering the rather substantial fee.