Is it possible to create a report to list all the nodes that a universal device poller has been assigned to?

You should be able to add the pollers during the discovery process (one of the tabs you will be brought to unless it's only when you use Add Node) if I remember correctly or add them via UnDP like you did before.

If you'd like to report on them, this SWQL query would return the caption of all of the nodes with ANY UnDPs assigned:

SELECT DISTINCT cpon.node.Caption

FROM Orion.NPM.CustomPollerAssignmentOnNode cpon

This one would return the same, but for specific pollers if you'd like to filter for that:

SELECT DISTINCT cpon.node.Caption, cpon.CustomPoller.UniqueName

FROM Orion.NPM.CustomPollerAssignmentOnNode cpon

where cpon.CustomPoller.UniqueName like '%<poller name>%'

Kind of just diving in with the SWQL queries  so if you haven't seen them before and have questions let me know.