I recently had satellite internet installed.  the techie that installed the satellite dish would not mount it on the SIPS roof  (10inch panels) next to our satellite tv dish.   He said he needed solid joists to put his lag bolts into.    I suggested that he drill some holes and insert some lead anchors and enter the lag bolts into them.   has anyone tried this ?   Or does anyone have any comments or suggestions on another method.

lead anchors won't work

try and keep out of roof deck entirely and mount on solid fascia board

I agree it would be better to stay off the roof entirely. If that is the only option my next suggestion would be to go with the largest toggle bolt you could find. Your installer was right. Given the surface area of a dish, a wind of any mention would probably pull a lag right out of 7/16" OSB. The toggle, spreading that force across a larger area might work. You should be able to find pull out strength/ratings for toggles on the internet.