I do not know the syntax of making a mksysb tape, at least you know how to make it using tar.
I hope it will help you finding your mksysb solution, else you at least have a backup solution.
If you need more help, please specify the exact command you perform, it might lead me a little closer to your solution.
If you are going to ask questions and then find an answer somewhere else, then please be complete and post the link to the (script) solution you found.
This site is to help *everyone* so please contribute your solution to help others.