Thanks for the input S! Yes that's right (in fact you could install more than one instance on the cluster on Win2003 as well but it was a lot more complicated and not recommended).
However, that's not my scenario. What you're describing is installing a 2nd SAP instance into the cluster - so as long as it's got a separate cluster service group you don't have an issue (cluster file shares can have the same name as long as they're in separate groups).
My scenario is installing a local (non-clustered/non-cluster aware) SAP instance using a local "sapmnt" fileshare. In this scenario, no matter how we set up the start + instance profiles for all the SAP instances, we cannot get them to reliably use the correct sapmnt share for that instance.
Ultimately we've had to install the local instances on a separate server outside of the cluster. Since that appears to also be the strategic direction that SAP are taking from an architectural point of view from Windows 2012 onwards (e.g. you have to install local SAP instances outside of the cluster if you want to use the new continuous availability feature), and since it simplifies the configuration dramatically, we think this is just a better solution anyway.
However, I'm still curious as to whether it's actually possible to get the scenario I describe reliably working even in the event of cluster failovers.