I've never experienced the SB system--for my sole Seabourn trip in the St. Lawrence River, I was able to take the train. All I can tell you is that Regent does have a flexible system, referred to as a "deviation". There is a fee for that, which goes up over time, but right now is $175pp I think. For that, you can choose your own flights and present them to Regent for approval. Some flights will cost an additional fee, which apparently can vary quite a lot (I've never experienced this situation, all my deviation plans have been accepted at no additional cost.) Until you and Regent come to an agreement, the deviation fee is not charged. You can open this negotiation 270 days before your flights (which usually means before your last fight.)
I find that Regent can do better almost always, in terms of price, than booking Business class international flights independently, lots better. Of course, if you can do it on points, you may do better than that. I also like that you don't have the burden of having flight cancellations to worry about, if your plans change (and of course doing it on points you're usually protected as well.)
As I said, I can't compare this to the SB system, but hope this helps. Regent has saved us big bucks in the past.