It seems like there would be a narrow window where firms would want a "Coming soon" website. So you'll only have like five minutes in the new firm's life get their attention and get them to sign on the line that is dotted. Also, since they are new they won't have much cash to spend.

There are lots of website creator websites (Wix comes to mind, or even wordpress), so cutting through the others' marketing noise and capturing a new firm within a short window seems very difficult.

It might be interesting to look at the tactics of capturing customers for other businesses that cater to the initial steps of creating an entrepreneurial venture. LegalZoom comes to mind (for incorporating a new business). Also, The UPS Store and getting a PO box --- how do they attract new small businesses?

My $0.02. Good luck!