If the signing is the same as the "Symbian Signed" process for Symbian 9, then you can install unsigned apps, you just have to say that you really mean it a lot, at least according to this:
Q. What's the advantage of having my freeware application Symbian Signed?
A. The freeware application - once it has been Symbian Signed - will get far greater prominence across the Symbian ecosystem and will install without an 'application source untrusted' dialog.
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Date: 2007-01-24 03:02 pm (UTC)https://www.symbiansigned.com/app/page/overview/freewareFaq
Q. What's the advantage of having my freeware application Symbian Signed?
A. The freeware application - once it has been Symbian Signed - will get far greater prominence across the Symbian ecosystem and will install without an 'application source untrusted' dialog.