Is it technically possible to change the mining protocol, so that a certain IP (solo miner or pool operator) cant have more than 49% hashpower ?
No.
#1 you cant force miners to update their client, so you are suggesting a hardfork
#2 you cant check how much hashingpower a certain IP has. You might allow only 49% of all block over a given time from a certain IP, but that would need a central place to deliver those blocks which can check where they come from. Thats not p2p, thats even more power on a single point. besides that: miners could just change the IP.
also: there allready is an emergency solution: http://gavintech.blogspot.de/2012/05/neutralizing-51-attack.html
Looks like this emergency solution is a push to centralization as already pointed out by some in the comment section. Old coins wont be there with new miners so mining will become the monopoly of the oldies.
Yes its a bad solution thats why its filed under "only use in case of emergency"
Old coins wont be there with new miners? I dont understand what you are trying to say.