Custom property as value in prevalue source

[bjarnef]

Currently when using Umbraco documents as prevalue source we can use Id, Key or Name as value.

However often it would make sense to use a custom value instead from a custom document type property, e.g. a school number or subject code on a course node.

[AndyButland]

Thanks. I'll close this though as a duplicate of #1195 (but will copy your title as think it better expresses what's being requested).