From a 100 degree C heatsink the air will move upwards from the heatsink, so as long as this hot air has somewhere to go and is not enclosed in a small space, you can likely neglect transfer by convection (i.e. heatsink warms air, air warms component). That leaves radiative transfer, i.e. transfer by infrared radiation from the heatsink to component. This can be hard to estimate, but depends on the emission coefficient of the heatsink (usually they are black for high coefficients), the absorption coefficient of the component, and how large solid angle the heatsink spans from the point of view of the component. This source of heat energy will be balanced by the loss of heat from convection (component to air) and conduction (component leads to PCB), and also radiation. Since a low absorption coefficient of the component means less energy absorbed from the heatsink radation, you'll want the component to be metallic (best case) or at least white. White materials and metals have absorption coefficients close to 0. One thing to note is that materials with low absorption coefficients also have equally low emission coefficients, so a highly reflective material (white or metallic) will emit very little of their energy as radiation.
All in all, this is a pain in the ass to calculate!
I just wanted to give you a physicists point of view of the process. My recommendation is to make sure the component is far away enough to not be affected by convection (a bad idea would be to mount it above the heatsink, in the path of the hot air - which flows upwards), and try to pick components that are reflective to minimize absorption by IR radiation from the heatsink. If that's not an option, put a metal shield inbetween the component and the heatsink to shield the IR radiation. Or, make the part of the heatsink "seen" by your component reflective. Since high reflectivity means low absorptivity, and low absorptivity means low emissivity. I.e., it will emit less of that pesky IR radiation.
Please bear in mind that this post was just based on logical deductions. If someone has real world experience, listen to them.
EDIT: One thing I didn't think of was heat transfer through the board. If the heatsink is 100 degrees, the board will probably be around the same figure, yes? At least nearby the warm object that is warming the heatsink. So likely a fair bit of the heat supplied to your component will come by conduction through the PCB or some power or ground plane.