I guess you'd need to use something like expect within the script (to wait for the passphrase prompt and then provide a passphrase in response on your behalf). However I don't think that expect is part of the n900 built-in shell & I am no linux guru (last serious *nix involvement prior to N900 was early 90's with SCO Unix), so not sure whether there are any other alternatives outwith writing some python (or whatever) code yourself to do it programatically?

Personally I don't think that the blank passphrase is much of a risk - sure, my server's public key is on my n900 ... but if that gets stolen I'll know PDQ if I can get it back or not. If not, I reckon I can disable my ssh server quicker than johnny thief can hack into my phone, sus my scripting and climb down my tunnel :-) Once I get a new device I just change the listening port and generate new keys.