Yes it definitely works I have it working right now and it's fine.
To be crystal clear for anyone re-reading:
You can run 4 MPUs (or two i2c buses of whatever you have either because of colliding i2c addresses or if you want better fanout) on a single teensy 3.1 (and I assume 3.0) I2C output, by switching only SDA between two possible pin mux options.
I use:
19 b2 scl
18 b3 sda0
17 b1 sda1
and I run SCL to all chips, pin 18 to one pair and pin 17 to the other pair (one module in each pair has address select changed so 0x68 and 0x69)
I found it beneficial to put 100ohm series resistors on the Teensy end but didn't capture a before/after on the scope to prove it, running 2m ribbon (i.e. from waist to hand/foot with extra) to each MPU (I used 3v3 power for the MPUs even though they have a 3v3 LDO on the module which I didn't bypass and they work fine)
I just hit the pin mux registers before calling i2c_t3 (after 'begin' of course) and switch just pins 17/18 pin mux registers.
Other notes:
I hacked in 'i2c complete' callbacks into i2c_t3 and did a simple i2c state machine for interrupt-driven MPU6050 polling (with no interrupt line to each MPU so only 4 wires total: power, gnd,scl,sda)
I set the DMP to update at 50fps (currently) and trigger a round-robin poll the MPU interrupt registers every 100hz to see if there's anything in the fifo, and read it.
800khz i2c works for me fine
Plenty of time for other stuff on a Teesny 3.1 @ 96mhz, I'm doing an 8x300pixel WS2812B strips @110hz with a modicum of RGB pixel processing (color waves, fades etc)
Teensy 3.1 is a
really nice microcontroller for small projects, I have to say; fast, capacious and modern yet definitely an MCU. The libs are quick and straightforward. It gets the job done really nicely.
In fact gotta get back to it ;-) I'll throw github up of the irq-driven mpu reader hack if anyone wants it but not right now.