No, I don't think it would be silly for you to have the ordinance number engraved into the Miyota case. The Laco Miyota B-Uhr is a modestly priced entry-level model, and to keep its pricing reasonable, extra features that add to a watch's production cost such as engraving its case are not included. If you wanted to have it engraved on your own, I think it would be pretty cool. In fact, I would send Laco an email first and see if they'll do it for you. I seem to recall having requested just such a thing a few years ago and they were willing, at the time, to take care of it for an additional cost.
There are other watch manufacturers that include the ordinance engraving on their models too, so you're right that it does help to complete the look of your watch. Let us know if you go ahead with it. You might set a trend in the process because I can't recall anyone having performed that mod before.
A few B-Uhr models I own that have the engraving:
