I look in dictionaries first of all, because a word that is listed by a couple of standard dictionaries can probably be accepted without my having to look any further. (A "couple of standard dictionaries" is not the same thing as a couple of dictionary websites, since a lot of those sites get their data from a few sources.)
However, a word that is absent from the dictionaries, as erging, and erg as a verb, seem to be, is not necessarily ruled out, especially if it is a relatively recent coinage, that may not have caught the attention of the lexicographers yet.
Erging is probably close to being too obscure, but I think it should be allowed. Even rowing glossaries tend to mention erg only as another word for ergometer, rather than as a verb meaning to exercise with the aid of an ergometer. However, these glossaries are not compiled by grammarians. The compilers possibly feel that the meaning of erging is obvious enough once you know that an erg is an ergometer.
A curious sidelight is that many kinds of exercise equipment are apparently fitted with ergometers, but the erg usage seems to have arisen only among rowers - not users of treadmills, exercise bikes, etc.
As you say, RM, there are plenty of examples online. "Erging music" gets lots of hits, as does "erging technique". Citations in edited publications are sparser. The Double-Tongued Dictionary entry that you link to has a quote from an Oregon newspaper. The New York Times has two usages (from 1998 and 1999). The Guardian has one, from 2007. Google Book Search reveals that the word is used in some books, such as Kayaking for Fitness.
Erged can also be seen - quite often on websites and very occasionally in books and newspapers. One example is from a crime novel, Hostile Environment by Patrick C. Crowell: "She flipped on the lights in the main bay, leaving the door open so she could feel the night as she erged."
I'll add erging and erged to the list.
