What about giving the netter a trident (spear that can parry)?
For Shield Bash, I think if anything it should have an increased likelihood of causing a less severe injury, since it's not designed as a weapon, and would suggest -1 to injury rolls (i.e. 1-3 knocked down, 4-5 stunned, and 6 out of action).
You could keep it at striking every other turn, although personally I tend to forget things like that, so maybe instead say it can be used every melee but always strikes last, and at -1 strength or -1 to hit? So it's a much less effective attack, but has a chance of catching the guy off-guard nonetheless.
Otherwise I like the idea a lot! Pit fighters really should have varied equipment.
EDIT: Don't let my suggestions discourage you, it's easy for fellow forum goers to say "hey, cool rules" and go on without posting anything. My recommendations are just that - not anything essential or necessary, just my thoughts. I do think that this is a genuinely cool idea, and the fact that originally I only posted potential changes doesn't show that I like the idea regardless. So, aside from any suggestions I made, nice job on some interesting house rules!