The code needed to check for the 'Donate' button would be minimal to say the least. That button is merely another UI element on the Nexus web page and it can be queried in exactly the same way as the 'Endorse' button.
The only difference being it is only added IF the mod author sets that option, thus needing a bit more code to check for its existence before querying its status.
A small icon added to the 'Flags' column, in the same way as the 'Endorsement status', would not be intrusive.
Exactly. The only issue is that the donate code will only be available for mods that activate it and set it up properly. Tannin could easily do a few sanity checks if there are markers in the API or wherever Tannin scrapes mod info at this time. There is only one string (the source ID for Paypal) that is required, so Tannin should be able to create a link in any manner he chooses using regular expressions, fe. to scrape the code and place it into his own context.
Why don't we add a donate button to the wiki mod pages then?
The point is to make MO attractive to mod authors by facilitating donations where applicable (which may be why Tannin added an endorsement button long ago). We could simply add some boilerplate text to all mod pages advising our users to consider donating to mods/projects that they use and enjoy. Similarly, we could add a reminder in the guide.
I was thinking more or less just linking the author's name to their Nexus Profile. If their Nexus Profile is set up to allow donations, you can simply click the donate button there.
It is still hard to find, and requires more clicks, which is less convenient and less likely to increase an MA's donation rate.
MO doesn't parse the nexus website because the structure of the page might change at any time. It uses a separate API intended for applications (primarily NMM). I'll see if that api provides a way to take users to the donate page.