What you are planning would be legal, VEX parts can be used pretty much any way you want to use them.
On a side note, back in Sky-rise, one of our high school teams used some old field parts for a counterweight (which shouldn’t have been passed by the inspectors) and competed with that counterweight all the way to Worlds (they passed inspection there, too). They always had a counterweight ready (made from vex metal) just in case, but they wanted to see just how far they could get. I guess the inspectors were counting the old field pieces as “non-functional decorations”
To add to that, you need to make sure the nuts and screw don’t fall out of the counter weight during the match.
If you want a lifting assist you could just use rubber bands. If you are worried about tipping over I recommend whealy bars (“Anti-tip pegs”). Generally, adding unnecessary weight is something you want to avoid.
Another word of advice is to add anti-tip bars rather than a counterweight since that will drastically reduce the overall weight of your robot while completing the same function. Of course using the counterweight might work better for you but judging by the description (clawbot) anti-tip bars would work better.