Signal reflections at the driver?
I don't think you would get significant reflections. If the load impedance is matched to the transmission line, there would be no reflections on that transmission line. In reality there's a tiny transmission line between the driver load and the long transmission line, so there's an opportunity for reflections there.

Think of it this way: "looking into" the transmission line, you will "see" a certain impedance (the wave impedance). Likewise, "looking into" the driver impedance from the driver itself, you will "see" another impedance.

Your driver transmission line is 15mm. The relative permittivity of most metals is 1, so we can assume the propagation speed is \$c\$. Your signal is 100 Mb/s, so let's assume a signal frequency of 100 MHz. The wavelength is thus:

\$\lambda=c/f=(3 \times 10^{8} \mathrm{m/s})/(100 \times 10^{6} \mathrm{Hz})=3 \mathrm{m}\$

What matters is the length of the tiny transmission line in multiples of the wavelength:

\$l/\lambda=0.015/3=0.005\lambda\$

This is too small to cause significant reflections. While you might get reflections due to the possible mismatch (as explained here, the reflection depends on the impedance values and not the transmission line length), the line is too small for the standing wave pattern to change.

The only thing you have to worry about is the voltage divider. Since the main transmission line is matched, you'll "see" it as a 100 ohm load: this will form a voltage divider with the driving load. For optimal power transfer to the load you'll want to minimize the driver impedance.

EDIT: If you want further reading, I'd recommend a textbook on the subject. I've been reading Fundamentals of Applied Electromagnetics by Ulaby et al., it goes over reflections and matching in detail in Chapter 2.

EDIT 2: I'm not in the field, I'm an EE student and we just happen to be studying this subject at the moment. I'm sure that someone with professional experience could give a more practical answer.