I, too, have noticed such hammer marks on wooden planes, but hadn't given it much thought prior to this question coming in. I can't really answer the specific question, but thought I'd provide a little bit of potentially relevant information which may help to start some discussion.
Smith' _Key_ (1816) shows two "hammers" which have textured faces. They are the Lathing hammer and the Brick hammer, though both have octagonal faces rather than round ones. I can envision a carpenter reaching for a lathing hammer to adjust his plane if it was closest at hand.
The 1845 R. Timmins & Sons Pattern Book also shows a wide range of hammers, and a few of them are illustrated as having textured faces. These include Glazier's Hammers (round and octagonal faces), Lath Hammers (round and octagonal faces), Brick Hammers (octagonal faces), Gardeners Hammers (squarish face), and Orange Chest Hammers (square face). None of the London or Lancashire pattern hammers are illustrated with textured faces. Based on this limited data, the most likely candidates would seem to be Glazier's hammers or Lath(ing) hammers. I'm thinking the latter may be the best bet.
If Moxon, Nicholson and Charles Hayward are to be believed, it would seem that the standard trade practice was to use a hammer to set, adjust and release the irons in wooden planes. I'd been taught to use a hammer to strike the iron and a wooden mallet to strike the wooden stock of the plane, so was somewhat surprised when I looked at these references with this question in mind. I mostly continue to use the hammer and mallet combination, but might have to rethink this if I were still working at a living history site.
Hope this has been of some interest.
Don McConnell
Eureka Springs, AR