Rothmans is all that there is in the public domain as far as I know Tom - any publication relating to RL before 1995 was quite a rarity. There was once a loose association of RL statisticians which I believe was led by Trevor Delaney but I haven't heard any reference to it for many years. Sadly all of the club's records prior to 1991 were lost in the Crown Flatt fire so your only detailed resource prior to that would be the RFl's own archive and the Reporter archive.
It will be a fascinating, frustrating and enjoyable exercise in equal measure - good luck to all involved.
I've been working on a similar project for Hunslet matches.
I think the statisticians referred to in the above post is the Rugby League Record Keepers Club run by Irving Saxton. They produced teams & scorers sheets for every club from 1895 to 1994. I have the Hunslet sheets & they're pretty comprehensive with just a few teams missing from the eary years.
They were until recently available on disc through the Rugby League collectors Federation run by Richard Bailey, although a recent request for a couple of missing sheets wasn't replied to.
If it'd help at all I could extract the matches involving Hunslet & Dewsbury - it'd at least give you results & dates.
Please reply if you're interested.