This is an idea purely off mental ranting, and something that I have yet to see. As we all know branding-free purchase is common on many types of software, however how about other than purchasing you could earn the branding-free license via referrals to XenForo.
Having the branding on forums allow XenForo to advertise itself freely and for users to know which software the community runs on. This in return ideally drives traffic to XenForo, but not necessarily guaranteed sales. If you guys roll out with something such as refer 10 customers to XenForo(ACTUAL SALES) then the branding free could be offered to that user. Incentive for owners to promote to the forum software, and extra sales for XenForo.
I have not thought this through by any means in business terms but again... simple ranting ;p.
I myself have no problem with branding on my forums at all, but this is just an idea.