Yeah, AIC sketch distribution is random, but 
there are a few patterns for the cels that do 
have their matching sketches.

There are quite a few of the really nice shin 
tenchi Ayeka cels having matching sketches.  I 
think animegame.com might still have some.

There's also a particular sequence of Tenchi cels 
from the 1st ep of the original OVA series that 
have their matching sketches.  I have the A1 cel 
w/ sketch and the others I've seen from the same 
sequence on other people's galleries have their 
matching sketches too.  It's a super close-up of 
Tenchi with his face taking up the whole field.

On one sales site alone (which has now closed 
down), I saw three Washu ova cels with their 
matching sketches.  A small full figure sequence 
of Washu in the first outfit she wears from OVA 
Ep. 9.  I wanted to get them, but as I said 
already, the site was closed for business.

Other than that, I've only seen 3 others.  One of 
which is a cel of Kiyone's spaceship (I used to 
own this cel but not anymore).  On rain's 
gallery, she has a matching sketch to a great cel 
of Ryoko & Tenchi.  Finally, I have a pan bg cel 
of Sasami & Ryo-Ohki with their matching sketch & 
gengas.