I have just taken a quick look into
ajax_combo.php
which is responsible for returning lookup-options.
I can
not find any resolving of placeholders in advanced queries' SQL of lookups (AKA:
custom_query
). So, I guess such a feature of replacing %%VARIABLENAME%% does
not exists in standard generated AppGini apps.
There was another forum question here a couple of days ago. There was need for joining a different table and filering by a value of the current record. Also in that question, the answer was: no, but there was a workaround for creating a custom dropdownlist with custom options.
This is the post I am referring to:
viewtopic.php?p=18751&sid=3ae9e6a20c544 ... 476#p18701
This one (see screenshot below) looks like a lookup (because it uses the same Select2 options as all the other dropdowns do), but is a normal
<input>
field converted into a select box with custom options:

- chrome_GSEHZ4asAa.png (4.01 KiB) Viewed 1282 times
Maybe this helps.
Another option could be the "
[x] Inherit access permissions
" flag, but, honestly speaking, I'm not too familiar with this. I hope others can help.