While you can use the same parameters from the message search in the UI with the message search in the REST API, the same is not necessarily true for the custom tags search since these are two separate searches.
However, it is possible to include metadata as part of the REST API message search... You can include it as part of the q parameter in the format "key:value". For example:
/search/messages?q=modbar.negativecomment:true
From there you can add in other search criteria (such as date).
/search/messages?q=modbar.negativecomment:true
this one works (for Attribute Tag)
But I have a problem with List Tag (another type of Custom Tag).
/search/messages?q=modbar.comment:negative
/search/messages?q=modbar.comment: positive
These two don't seem to work if I set up Custom Tag as List Tag.
Hmmm... There's an alternate method for specifying metadata criteria when searching that might work better with list tags. You can specify the name of the metadata field as a query string parameter, and the value of the metadata as the value of that parameter. For example:
/restapi/vc/search/messages?q=date:[1341341612 TO 1341946412]&modbar.comment=negative
