The current implemenation plan is:

1. Create a new WSDL entity to hold the filter values. These should be all the values from the IssueData;
2. Create a new SOAP method which takes in a username, password and the filter values;
3. In the method implementation, unserialize the filter and apply the search using filter_get_bug_rows.

Skeletal implementation started at https://github.com/rombert/mantisbt/tree/soap-custom-filters .

Here is the pull ruequest for this work:
https://github.com/mantisbt/mantisbt/pull/560