So on paper this looks good-and I'm sure someone can make this work.
But as I read this this morning, and today happened to be ARRL Winter Field Day, so we tried it today. It doesn't work worth a crap compared to RG58, which is what some in this thread have been comparing it to. We used a 100ft length.
A matching balun on each end, to convert from 50R to 100R impedance, feeding a dipole antenna at 72 ohms feed point impedance-exactly the same configuration as if the dipole was being fed from a 50R coaxial cable.
With the RG58, signals were coming in fairly strong-there was no shortage of contacts coming in! We were a bit weak on transmit due to feedline loss, but not terrible. Normally this run is fed by RG8U.
With the CAT5e, using a single pair, it was as if we were hooked up to a dummy load. I think we heard 1 or two very weak calls come in, and we could not get any kind of power out. Couldn't be heard even across town.
As said-I'm sure someone can make this work. But spending a bit for even some RG8X will be a significant improvement in radiated emissions, power handling, and feedline loss.
Assuming the holes are big enough outside, You could use your cable, tie on a pull string, and pull the cable back till the string is accessible outside, then tie on your coax, and pull both the coax and the CAT5 back in. That way you do not have to drill any more holes. If the hole isn't big enough for both cables, you could just pull the coax back in, and leave the CAT5 pulled out-just make sure you pull in a new pull string with the coax. This way, in the future you can put it all back the way it was with ease.