It is not an R18 exception you want (withholding), it is a R16 exception (bag and tag rules). With no robot in a bag with a correct seal number, you will fail R15/R16.
What i would do is email First HQ with the following:
1) Explain the situation
2) Include a copy of the police report, or if that is not yet available, the report number (proof of event)
3) Explain what you would like to do:
a) use the practice robot for your Robot
b) Perform the following modifications in order to make it functionally identical to the robot that was bagged
c) Bag the robot by <date>
d) Use the following withheld parts for the R18 rule.
4) Request an exception to R15/R16 for the above due to the unusual circumstance.
Assuming that First e-mail's you an exception, I would then print out your original e-mail, First's email giving you the exception, and attach that to your lock/unlock form that has the current tag number.