I also always leave the country after extension before my 90 day report is due. Last year after re-entry I did my 1st 90 day report in person. But this year after re-entry end of September I registered for online reporting and just today did my 1st 90 day report online 14 days before it was due. It was approved in about 3 hours. So it therefore does not need to be done in person every time after re-entry.
If you look at the 1st page of your IDP (p.3) you will see the categories written in English (p.8 for Spanish), and A is for motorcycles so no you are not covered.But hey! if you dont know, Im damn sure the Thais renting mbikes (nor the Thai police) would not know either. I think that you would only have a problem if you had an accident and needed to claim from your home country insurance. BUT if you have no experience of mbike riding then forget it as Thailand's roads are VERY dangerous; Thailand is #2 in the World (probably #1 as only deaths at the scene of the accident r counted) for road accident fatalities and over 70% of these r for mbikes! So, 'EVERYONE', take care out there.
I mean generally: Visas r now issued online, 90 day reporting can be done online. More and more will be done online, as with everything. So not so many 'jobs for the boys'.