Ok. I thought you meant a I2C bus expander like:
http://www.ti.com/product/pcf8575

But you were thinking about a buffer chip to allow the I2C signals to be routed a longer distance.

So actually not a I2C bus expander, but a I2C bus extender. Something like:
http://www.nxp.com/#/pip/pip=[pip=P82B715]|pp=[t=pip,i=P82B715]
New link - above not 8052-safe even when using 'Link' button:
http://www.nxp.com/pip/P82B715.html
<RANT>Thanks to web developers who do not read through the HTTP standard regarding valid characters in an URL. All "trial-and-error" and if the web browser finds the page, then it must be correct.</RANT>

A question then is if there would exist any such buffers that would support 5MHz, which isn't exactly a normal I2C data rate.
And if such a bus expander have one clock and one data line, I would probably need around 70 chips, to get 70 auto-reversable data lines.

Just looking at the P82B715, Digi-Key claims $1.60 at 10k volume. I wonder if that route would allow me to get below $0.50/signal or $35 / board even if scanning all potential chips.