I apologise from the beginning if I write or ask something obvious or stupid (hopefully not obviously stupid), but if installed in a device not connected to the Internet may work as an offline signature system?
Is there a way (qr code? bluetooth?) to send the signed transaction to an online machine to be broadcasted?
Or that maybe can be a separate implementation done using the command line version...
Happy you asked.
You could sign a transaction via keystash gui or command line on a raspberry pi or notebook without internet connection for example.Output file is for example: 2014.12.19_10.40.00_PM_MEZ_recipient_NXT-FUND-3HGT-4RU4-4AQ89_amount_5.tx
You can broadcast and read the transaction bytes for example with official NRS.
The advantage a trezor has is that you have some fancy push button method with a nice and shiny device that is doing offline signing(private key is not leaving the device).
With keystash it is up to you how you transfer the signed data .tx file to a system with internet connection also what devices you prefere to use.
Anyway Nxt has no market for a trezor(you probably could sell 100 to the community) and bitcoin it looks like a trezor will be more worth than 1 bitcoin
I would say keystash is a mix between keepass, armory, trezor and a classic wallet.
You can try the keytash signing process with test Nxt or a little amount real Nxt its fun