Fixed it by using the MakeImageWriteOption node rather than splitting the pin on ExportToDisk.
The Export texture node seems to be broken. Using the Create Event node for the Options.onComplete delegate parameter results in not being able to select a signature, as the dropdown box is never summoned.
Using the delegate node from a custom event instead results in the error, “failed building connection with ‘Delegate is not compatible with Delegate.’ at Export to Disk”.
Any idea how to fix this?