Hubert,
Perhaps some of the developers will see this and consider the gap you identified. However perhaps in some way you are asking to redefine what an orphan is, although it may make sense, perhaps it would have a performance hit only for a few cases. Perhaps simply build your own filter to find the tiddlers you are looking for
I imagine what you need to do is to use the filter in $:/core/Filters/MyOrphans
[all[orphans]!has[tiddler-type]!tag[Journal]sort[title]]
and perhaps extend it with a search operator with the fieldname suffix, and negation "!" to check for the non existence in your additional field(s).
Regards
Tony