I've been using 1.9.8 (not a portable version) on a Tails USB key. You can funnel it through Tor nodes, but it requires for some kind souls to keep an Electrum server up on a Tor node. Unfortunately the .onion/Electrum servers seem to be infrequent.
If you use the -1 switch, it keeps it from trying other servers.
I consider this to be very secure. You could be on the most infected computer in the world, and it can't touch this. A hardware keylogger would be the only possible way to lose your passwords, and Tails has several virtual keyboards or Keepass that will defeat that.
Malware could replace the electrum.exe with one that reveals everything. I suggest not to be uncautious.
Why do you need to use onion servers? By using tor you still can use all normal servers. Or do you want to have a server whose location is unknown to authorities?