Lookup filtering

Hello,
I have the following setup:
Table 1 with the following:
id|name|licenseid|other columns
1|test|1|…
2|test2|50|…

Table 2 with the following:
id|licensename
1|1LIC
2|2LIC
…….
50|50LIC

I created a lookup rule on table 1 to show licesense name instead of licenseid, and also I have created a new view to only show the table2 entries that do not have anything associated in table 1.
I have no idea how to acomplish the following:
When you edit an entry from table 1 in the dropdown for licenseid you should only see the entries that are not associated to anything in table 1.
I have tested the following cases:
1. the lookup is done agains table2 – everything is ok in the “list table” view; in the “data entry” form I see all entries in the dropdown
2. the lookup is done against the view – in the “list table” view I see the actual ID’s (since the licensenames are not matched to anything); in the “data entry” form I see only unassociated entries (this would be good), the problem in this case is that the entry initially shows up as blank and if I save it like this the entry will actually get updated with an empty value.
3. I’ve added 2 lookups – one against the table and one against the view and somehow I managed to get the licensename in the “list table” but I still have the issue with updating a row (the current “licensename” does not show up in the dropdown, and if I edit any other cell and save the “licenseid” field is olso updated to empty).

I hope I managed to explain the situation… can you think of another way to accomplish this ?

Thanks,
Stefan