I'm working on a raspberry pi project that will use about a dozen i2c devices (mostly mcp23017s) and the wiring is getting a little crazy. Are there any downsides to routing SDA and SDL to one of the power rails and using them as a sort of bus?
i2c is time dependant. Most devices have limited pull down capability so that limits the size of the pull up resistor you can use.
Using larger pull up resistors with additional capacitance from the additional devices will limit your rise time.
Your rise time will limit you data rate.
So be sure to understand what is the smallest resistor value all of of your i2c devices can use, what is the total capacitance you have from all the devices, and there what is the fastest speed (or working from desired speed maximum number of devices).
Using the rails on a bread board will increase your stray capacitance, hence limit your rise time and speeds.
You can always check your rise time using a scope to ensure your timing will not be effected.