Could not the Kaioken be very different from going SSJ? It's seperate from a transformation or a typical powerup, observe:
{Saiyan Saga (ESF Vs. Vegeta and Nappa) Spoilers}
*Quotes aren't word for word at all*
When Goku first uses it to take out Nappa, Krillin's like "If you could do that, why didn't you take him out right away."
Then Goku said, "Well, there's a small chance the strain could've destroyed my body.... the Kaioken can double my strength and speed... but it only lasts for a short amount of time."
Goku then tried going Tripple and Quaddruple Kaioken in his desperate attempts to kill Vegeta, and after the fight was pretty damn spent. His body felt "torn apart," etc. etc.
{End spoilers}
So, the Kaioken could be a short-timed huge increase in power that either does major damage, has a % chance of killing you outright, or both. There could also be triple and quadruple Kaiokens, but those should either weaken you to a "Ki blasts can kill me!" level, or have a very large chance of killing you.
It can be added it as a seperate, desperation powerup, but one which freaks people out when it happens. Could also add a bit of interest to powerstruggles... Assuming it could be balanced.
Edit:
(Lemme know if anything I just said is wrong - that's all dubbs info.)