I was under the impression that this kind of interpretation of employment contracts was more or less illegal (or, at the very least, unenforceable) in many states (I'm assuming you're US, programming in Fortune 500, so maybe not applicable?). Sort of fits into the same kind vein as certain non-compete clauses, where companies put them in, but they'd be tossed out pretty quickly in court should things actually reach that point.
If it's not on employer-owned equipment during core working hours, then I believe it's pretty difficult for an employer to enforce that kind of thing. I don't know how important to you working at that charity was or participating in open-source now is to you, but if you're willing to take a harder line of telling rather than asking, you might find that when push comes to shove, there might not be much they can do without drastically escalating the situation (not always an organization's favorite thing to do, even if they can technically win, which they might not be able to).
Obvious disclaimer about being a non-expert on this sort of thing, but really just wanted to point out that the first answer in this situation may not actually be the final one.
My recollection is there was a compulsory agreement saying you would not do anything of a nature related to your work, or if you did, allow the company to decide if they wanted to claim it.
Also, running side businesses was not allowed.
Of course, the normal thing to do was just ignore the rules, because most likely nobody would notice or care. I explicitly asked for permission because I wanted to see what would happen.
If it's not on employer-owned equipment during core working hours, then I believe it's pretty difficult for an employer to enforce that kind of thing. I don't know how important to you working at that charity was or participating in open-source now is to you, but if you're willing to take a harder line of telling rather than asking, you might find that when push comes to shove, there might not be much they can do without drastically escalating the situation (not always an organization's favorite thing to do, even if they can technically win, which they might not be able to).
Obvious disclaimer about being a non-expert on this sort of thing, but really just wanted to point out that the first answer in this situation may not actually be the final one.