What I hear from one of our application specialists is that there is no accessory that will help and all you can do is minimize the shaking of the vial just prior to sampling the headspace with the syringe. It sounds like you have come up with an ingenious work around, unfortunately there is no accessory to help.
I think your workaround is appropriate one in this case as there's no specific accessory for it. Also as Martha mentioned you want to minimize shaking of the powder.
Other things that could also help is to use custom vial pressurization and loop fill parameters. Even when headspace syringe has hole on the side high flows/fast pressure fills may create vortex and move fine powder around the vial. Then when moving sample from pressurized vial into loop too fast may suck material into needle/loop/valves.
Checkout these settings in HS parameters:
I went ahead and flagged the “Correct Answer” on this post. If you still need help though, please let us know and we would be happy to continue working with you.