If Spike's busy having adventures with Brian Lynch at the same time season 8 is happening, then it'd be harder to fit in an appearance.

It may not be the same time. If Lynch uses the end of Angel: After the Fall as Spike's jumping-off point, he'll be starting the series immediately post-"Not Fade Away." That means Spike's new series will be set 12 months after the events of "Chosen." BtVS Season 8 begins 18 months after "Chosen." So the two series may not be concurrent.