You wanted the page on
Custom Tags here, but given that you've tried to solve it yourself, I'm very happy to help!
In the "Define New Tags" dialogue, you want the top section. Under Identifier, put Release Date. The other fields will now populate, but you want to replace the id3 field with RELEASETIME.
Now exit that dialogue and in the Custom Tags part of Preferences>Tags (1), fill in a custom tag of your choosing with Release Date in the Display Name and the Save To box.
EDIT: That's the process you have to do for your custom tags normally. However, I've just flicked through the MB default options, and Release Time is in the drop down normally. Try setting a custom tag to that first.
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