Yes, you can put this own 'code' for example into the 'Comment' or 'Grouping' or any other field - I don't understand the question.
The program (ProppFrexx) will of course not directly understand 'your code' (as nobody and no program will automatically understand it) - but you can use your code for example in the Filter Criteria as explained in my post above.
However, I don't even understand what benefit your code should actually bring. Instead of using one 'funky code', you can better use the individual fields by it alone

E.g. you can directly specify and use the fields separately (Artist, Genre, Mood, Lyrics etc.) even in your Filter Criterials, e.g. you can use the Filter to specify things like:
GIVE ME ALL tracks matching the following:
- 'Artist'' must start with 'A'
AND
- 'Genre' must contain 'Slow Jam'
AND
- 'Mood' must be equal to 'Happy'
Any logical combination of any meta data field is possible in ProppFrexx - so there is not even the need or advantage to use any such 'code'.