Cheers for the response. What if we look at it from a radar point of view though?
Say for whatever reason your colleague in the tower wasn't happy with providing the separation (it's a bit misty). Only 2 of the 3 reduced sep. rules are now left to play with. The IFR lander is with tower and the SVFR crosser is with you on radar, visual with the other and happy he can keep his own separation. If the "following" rule is legitimate for this scenario surely this would be the most expeditious way of getting him across without having to phone tower to check with the IFR if he's visual and happy or waiting till he's landed?
Obviously it's better if they're both on tower's frequency for them to sort out however I've experienced a few tower controllers reluctant to work a crosser. Hence my desire to try and get the rules fully understood!