RIPE Atlas can be useful for testing (my node just hit its 5th birthday!), but this might be something better tested with random clients. If you can convince someone running a speed test site to include a .0 test, that may be a way to get a little more coverage; or if you have budget, you might be able to make something work with web ads that beacons to a .0 and a .something else, and count the returns, etc.
EDIT: I hadn't seen the EC2 reachability test; it's got some nice test addresses like 100.24.0.0 and 107.22.255.255, so that seems pretty comprehensive. It would be interesting if someone at AWS can share statistics on if there are significantly fewer images loaded from .0/.255 addresses than otherwise; and if they can narrow that down by AS or otherwise. Publicly would be great for my knowledge, but at least sharing with the ietf would be useful for decision making.
http://ec2-reachability.amazonaws.com/
which puts many of the test hosts on .0 addresses.
There was also a NANOG thread talking about this phenomenon.
Decades-old standards do call for interoperability with a distant host whose address ends with a zero octet, so in theory there should be some interest toward cleaning this up on that basis alone.
If you have some more data or examples or suggestions for where to find them, I'd be happy to hear about it.