@zoeck Being physically able to do something is not the same as legally/ethically being able to do something. I haven't found the license that pro modules are provided under, but Ryan makes it clear they are not open source, so your rights are unknown. Common sense says that I'm quite sure he doesn't mind anyone making changes to their local copy but that I'm quite sure he would mind if someone started sharing the code/modified versions of it, or even selling it!
My interest in open source code is primarily sustainability - code that is openly shared/forked, has good leadership as part of an active team of developers, welcomes contributions. A closed product only exists as long as it is in the interests of its owner. If Ryan, God forbid, were to have a change of fortunes/interests, and shut shop, the open source code could be picked up by an interested community of developers and all our websites and services based on PW stand a chance of a future. The closed source products could not - legally - be continued. My biggest reservation in using ProcessWire is the business risks posed by its BDFL model / single point of failure in general and moreso with the pro modules.
My interest in this forum post, however, was just how to give Ryan a modest donation to help fund his excellent continued work! I'll try reaching out directly/via contact-form on website as @Krlos suggested.