I understand not wanting to get hooked on xanax but what is your plan as far as how you are going to use the benadryl to avoid this possibility? How are you taking the xanax currently? On a regular basis? Are you looking to supplement with benadryl so as to not have to increase your dosage or to alternate between the two in order to lessen the frequency with which you're taking the xanax? Because if its the former it sounds like you are already dependent to some degree and simply not looking to escalate that dependency by avoiding or at least slowing down the 'need higher dose to achieve same effect' track of progression that benzos, especially a short acting one like xanax, can place you on. And that's certainly a valid goal.
Without knowing your current dose/tolerance I hesitate to comment with authority on the safety of the combo but they both increase sedation so you have to exercise the same caution you would with any similar mixture. That being said, if you're stabilized on 1 mg of xanax e.g. not nodding out or getting jelly-legged, adding small quantities of benadryl should not be an issue, but wouldn't add a third substance with sedating properties like alcohol to that cocktail.
'As human beings, our only sensible scale of values is one based on lessening the agony of existence. That plan is most deserving of praise which most ably fosters the creation of the objects and conditions best adapted to diminish the pain of living for those most sensitive to its depressing ravages.'- H.P. Lovecraft