I2C & SPI - share clock lines?
You could theoretically do this but you'd have to use two different buffers. The system I'm envisioning would have a bidirectional buffer with tri states might work (or maybe two) and you could switch the SPI bus on and the I2C bus off. 

The pull ups for I2C could be on the other side of the buffer

However if one is using a small micro, they are probably doing so to save on space and you might as well use a bigger micro than a micro and a buffer (or two).