I can not really say that it is deliberate or that I had considered the seyan vow of poverty issue.
You can add it to the trading post and it will remain and at this stage I do not have any plans to change that, in spite of the possibility of abuse.
Items on the trading post are for sale and anyone can buy them at any time. If you add your valuable items to the post (because you are going seyan) and soeone buys it then so be it.
Most people just give them to someone they trust to hold, and hope they do not get scammed (which is also a risk).