Although I know now that I should have had a "permiso", at the time I didn't (since found out on an ex-pats forum), and no-one thought to inform me.
2013 - I borrowed a bike and went to get SOAT from Positiva - all they asked for was my passport and the bike papers (in someone else's name!) and sold me insurance.
2013 (later) - I looked into buying said bike. Went to a large Notario in Trujillo, who looked at the papers and simply told me the vendor had to be there as well (he was back in the USA), so that fell through.
2014 - bought a bike and was told to go to a Notario with the paperwork. Again they did not ask for any "permiso" (still unaware I needed it) and authorised the documentation.
2014 - went again to Positiva with the paperwork I had and they gave me "temporary" SOAT, which they told me was all I needed along with the bill of sale to drive legally in Peru. When I got the tarjeta and plates I went back and got the SOAT changed.
2015 - again went to Positiva and bought SOAT for this year - only bike papers and passport requested.

So I "cheated the system" but since no-one (different clerks at Positiva and Notario) informed me what "the system" was how could I have known what the correct process was? You have to trust people in authority to inform you what you do or don't require, and no-one indicated that I was doing anything wrong. OK, ignorance of the law is no excuse, but my "cheating" was simply trusting people who sell insurance or sign documents.