Mats,
Unfortunately, you're hitting the limits of anchored frame behaviour.
It's an either/or proposition. You can have run-arounds, but you can't
control the position except for placement at the start or end of a
paragraph.
An alternative, but clumsy and labour-intensive, is to set the start
positions (indents - first & left) of all of the document paratags to
be the desired offset (1-2mm) of the anchored frame. Then by setting
the frame to start at the beginning of the anchoring paratag (not set
with the indents), you will get the equivalent effect.
If this is just going to be a single location and won't be subject to
much editing (i.e. document reflow), then you could also consider
using a graphic frame set with runaround, which you can position
anywhere.