I've been trying to add support for image viewing modes on my site.
(I.E. while viewing an image, a user clicks a button that changes the viewing mode to album, single-image, or all at once view.)

One of the modes I would like to incorporate is horizontally scrolling all images of an album on one page.
Here's an example of what I would like to have.

I've been able to create this mode by hacking the Slideshow plugin, but I would like to make a custom theme page that does this without hacking. For example, going to [i]tedph.com/album?p=scroll[/i] or [i]tedph.com/album/scroll[/i] would pass on the album to scroll.php and output all images of the album on one page, in scrolled form.

No need to hack the slideshow, that is possible even easier: Just replace the printImageThumb(getImageTitle())within the ´next_image()loop on album.php withprintDefaultSizedImage(getImageTitle())` and you get the big images instead of the thumbs.

Just as a note: Regarding usability horizontal scrolling is really inconvenient, even with a mouse that supports that. I won't look that much on your site with that. But just my two cents.

Is there a way for me to use your method without having to change my album.php file?
I would like to have the album view available as well as a scroll view, so accessing the scroll mode will use [i]tedph.com/album/scroll[/i] instead of overriding [i]tedph.com/album[/i].

Regarding your usability concern: scrolling view won't be mandatory, it is simply an option for users that prefer to view the album like this.

you can create a copy of the album.php file and name it scroll.php and make the changes there. To link to this file use (mod-rewrite enabled) http://yoursite.com/album/page/scroll where album is the name of the album you wish to display.

Will I need to modify my .htaccess file or do the ZenPhoto mod-rewrite rules automatically change the link?

Never mind about that last question. Yes, ZenPhoto automatically does this.
Thanks for the help, this was by far the quickest fix I have ever made, and now I can update ZenPhoto via svn!