If I’m charging businesses for a text messaging service, could I include an absolutely free power ball lotto pool entry per text in?

This question require a great deal more details than afforded on a Q&A website. First, you are highly restricted in sending advertising messages as part of an SMS without direct consumer consent. Just because your clients may permit it, does not make it compliant under FCC regulations. Second, assuming you can run the sweepstakes promotion on your SMS system, the key question would hinge on who is making the payment. If your clients are using the system to enable usage by their customers then its possible that an analysis would show that now "entry fee" would be required of the customer. If your clients' employees are entering the sweepstakes, it's possible that there is an entry fee. There are just too many variable to determine if your plan is legally permissible.