use free tools like rawdigger ( http://www.rawdigger.com/usermanual/selection-grid) / argyll ( argyll has 3rd party GUI front ends like CoCa = http://www.muscallidus.com/coca/ or MakeInputICC = http://sail2ithaki.livejournal.com/191061.html ) to build icc/icm profiles
*** note that C1 wants LAB PCS & lut in AtoB0Tag it you want to use internal color editor in C1 later (that means that a custom icc profile which is pure matrix only + cieXYZ PCS will not be accepted by internal color editor in C1 if you decide to fine tune color inside C1 itself, even if it works just fine for raw conversion there) - at least that was an issue reported by people quite recently.
You will need to process the image in Capture One first, and select "Embedded" color profile. This will apply all the pre-processing steps, including non-linear curves, to the image, but the ICC profile will not be applied (it is embedded in the file). You want to create a color profile that replaces the embedded ICC, so the profiling tool needs to ignore the embedded ICC profile.
An image with an embedded profile will also contain the now-standard TIFF tag ProfileToneCurve (to go back to linear space). In theory, this should make it a much easier job for a profile creator, but they must obviously be aware of that fact. It could even require a different set of algorithms, depending on the exact way the software builds the profile.
It does require some fiddling to get this to work with tools I am aware of. X-Rite did a good job of making the process easy to apply for the supported Adobe products; it is not a simple thing to do from an engineering point of view. The quality of the resulting profiles are, I suppose, a subjective matter.
Oh, and Capture One supported XYZ as a PCS for some time now. It does require the 'A2B0' table to be there, so a pure matrix profile will not work. Caveat: that feature is not tested thoroughly, so please knock yourself out and report any issues ;-)
Esben H-R Myosotis