Not a good idea for two reasons I can think of right off the top of my head.
1)Sitting in a car, urinating into a condom cath probably will not have enough power to get the urine high enough into your colon to stay there. Which would cause the urine to back up, possibly causing the urine to come out around the condom cath and into your pants, thus, making a mess.
2) Again, you are sitting down, gravity is not working with you here. So, even if the urine doesn't come out of the top of the condom cath, the urine will still be entering your colon, then backing right back up into your urinary system.
So lets see, in about a week or less you will have all kind of things going on, it will burn when you pee, also may hurt like hell, you may even have dribbling going on, or frequency just to name a few, also may have severe abdominal cramps. So, you will end up either at your doctors, or the ER, where they will probably cath you for a urine sample, and might even do a urethral swab, which can be pretty uncomfortable if you are having issues in that deparment, which you will be. Then it will come back that you have a raging urinary tract infection caused by E-coli. Then you will be handed two pieces of paper, one will be for a presctiption of probably Bactrim DS, or cipro, and a bill for about 750 bucks. And when the doctor comes in to tell you what you have in your case because you are a guy, it would be a bit different, a women he would tell her to either remember to wipe frot to back, or to make sure during sex the guy changes condoms between holes. But since you are a man, he will tell you that you need to make sure you wear a condom during anal sex and then he will hand you a bunch of pamphlets about sefe sex. Or, you could just tell him you put on a condom cath, stuck the other end in your butt and went for a 6 hour drive, lol.
Good luck with whatever answer you decide to tell them.
Seriously though, probably not a good choice.