Asking for details of an algorithm developed in an article in review
In a similar situation, I have sent an email to the editor asking to pass on a message and giving permission to reveal my name and contact details to the authors if they wish to respond.

Alternatively, but more problematic, you state that you may be able to replicate it. You could go ahead and do that and by the time your work is complete, the original article is likely to be available to be cited. If you were to go down this path, though, you should still be discussing this with the editor as part of your agreement as a reviewer is that you don't do this.