Hi Neil,
In general, this isn't the safest approach as we add new capacity all the time and may end up not being able to add it in our existing subnets. Because we consider the IPs to be internal we don't really notify of changes in that regard. If you want to dig into this more, I'd recommend signing up for the Launch Point developer program (http://developers.marketo.com/support/launchpoint-developer-program/) and asking about it during the office hours.
-Erik