Is there a way to define and work with a Dictionary meta property type with the data annotaions in 7.5?
I'm afraid it's not currently supported.
However, it's actually one of the features we are working on right now. If all goes acording to plans it should be available in a nuget pakage near you in a few weeks. With the usual caveat that things may change, plans may fail, etc.
Currently Dictionary (Single value/multiple values/String) property is not supported in Catalog content in 7.5. We have a plan to add support in the future releases.
Oops! Per already answered this question.