But it should be possible!
As far as I remember, it has been possible in one of the earlier versions.

2013/2/24 Holger Marzen <holger@marzen.de>
On Sun, 24 Feb 2013, Stefan Thomas wrote:

> Dear community,
> I'm using rosegarden, version 12.04.
> I have the following question:
> Is it possible, during a recording, to let the metronome 2 bars count,
> before the recording starts?
> I've been searching for this option, but I didn't find it!

Neither did I. I start recording some bars before.