Welcome! It sounds to me that it would be (fairly) safe - the RAMDAC won't be able to do the right thing, because that current isn't going to be constant, but as you say, it looks like it won't exceed the spec too much. I think I'd choose 1k though, to be safer. 5/560 is already 8.9e-3, and the resistor and power supply could be off by 10% each, giving you 20% over.
You also need to be sure you've picked the right value of resistor - test it! Those colour codes are easily misread.
If you don't have 1k, a pair of 560 in series will do.
Note that you don't know what that input pin is going to do, voltage-wise, so you won't be getting a constant current.
It's an interesting challenge perhaps, how to cook up a constant current source from things you might have lying around...
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