Allright,
I've got news, but it's still not white or black:
I phoned with somebody from the Iranian Embassy in Belgium (acquaintance of my father) and explained him the situation. He knew nothing about it and thought it was bullsh#t but would have a look at it. He phoned me half an hour later back with details of a Dutch agency that handles these kind of stuff as they are better informed than them (

)
So I phoned the Dutch agency. After half an hour of talks (aaargh my phone bill!) this is the result as they told me, hard facts (for you tee bee

).
IF all your
documents are
100% ok, this means your passport with the
visa AND all the required paperwork for the bike/car (including the
CPD!) you
should have
no problem getting in.
BUT, they clearly told me that
it is not because you have alle the papers that the borderguard is obliged to let you in the country.
And thus one of the things they could use to refuse you (if he has a bad day or so) is that you need a guide.
They also told me that last week (!) a group just went from Turkey into Iran without any problems and without a guide.
I also asked them about the fact that Iranian tour operators are now telling everybody that you NEED a guide. Their answer was that Iranian companies don't want to take responsibility for the fact that you MIGHT get refused at the border.
Which I can understand and which makes sense. Reason why they are telling everybody that you need a guide, to be sure.
So, after all these phonecalls this leaves me with the following thought and conclusion; one needs to have all the documents (and pay for it) and then once you show up at the border you still have a chance that you're not allowed to get in... but then this is the case at almost any border, no?