If I understand Brunson's study conclusion correctly "Attempts to backcross male bluegill x green sunfish hybrids with females of each parental species were unsuccessful both in ponds and in the laboratory", the male 'standard' HBG will not cross with female BG nor GSF. However we do know that the male & female HBG will successfully produce offspring, thus the need for predator control in a HBG pond. Brunson's conclusion of "The reduced population fecundity resulting from the low percentage of females results in low recruitment of F2 hybrids and consequent avoidance of overcrowded conditions." is not absolutely correct. I think the use of "consequent avoidance of overcrowded conditions" was a biased statement and somewhat exaggerated by the author; reduced chances of overcrowded yes, avoidance no.
So with the HBG being able to reproduce among themselves it is very possible to have a year class of mongrel HBG (F generation) each year, thus production of a F4, F5, F6, etc individuals. I am not aware of any detailed genetic studies of the offspring FX generations from HBG. FX meaning subsequent backcrossing of the hybrids.
To read about Meadowlark's experiences with FX generation HBG see information in this link:
http://www.meadowlarkponds.com/TGG.htm 
