Right. I don't think there is a way to do that. We expect you to pick the object corners, then we set how far from that point we start drawing the extension lines. Short of point editing each dimension after you create it, I don't think it can be done easily.
Rhino 5 does not offer this option. This is a constant length extension line, and does not extend to the extension line origins. However, I will add this to the wish list for consideration in future Rhino.
this issue was brought up in the rhino newsgroup:
Would it be so difficult to just offer an additional dimension-arrow, that includes a short vertical/horizontal dash (while having the "suppress extension line" option checked)? Is it possible to create a custom arrow or modify an existing one?
Also along these lines, I would like to be able to change a dimension's line styles from solid to dashed or dotted. There doesn't seem to be even a manual way to do this.
There is no options for this control in Rhino yet.
But I think it would be useful.
I have added this as feature request, together with the fined length extension line.
Thanks for letting us know.