There was a trick on N810, maps up/down on volume keys or home/menu keys to scrolling in browser.
When one is browsering, I think the most useful feature is scrolling down, such as reading this forum, you zoom only once and most of the time is scrolling down.
Touch gesture is nice however not responsive enough. Volume keys is mapped to zoom....

Any trick or configuration on N900 too?

So what's your exact request here? A hardware button to scroll, while the keyboard is not shown?
Taking a look at the N900 here I don't think there's many such buttons available in that case, so you can draw your conclusions... :-)

There is no such button on N810 too, however, with a bit config change, I can map keys to scroll.

I suppose the up/down arrow keys on the keyboard are not what you want?

They seem to work for me. A pgup/down might be handy - ctrl or blue arrowkey + up/down arrow maybe?

Edit: Sorry, missed "without open keypad"...

Obviously, if you don't use the keyboard, then you are limited in what you can do with the remaining buttons. My bad.

just use light pressure with the side of the index finger,such that the finger nail just makes contact , scrolls very smoothy and responsive.

I know he mean "without open keyboard" but if you want quick pgup/pgdown try Spacebar (pg down) and shift spacebar (pg up)

Sound volume key +/- is now mapped to Zoom in/Zoom out in browser, I am sure there is a way to change mapping to up/down arrows. Just need to find the config file. I have checked about:config, seems not there...