[DNSB]

For what it might be worth, you can have omnibus numbering use [1-3] easily enough for Kobo devices. There was a discussion on MobileRead about that a while back with, IIRC, Charles Haley doing the heavy lifting. I do use the series with the series number set to [start number].[end number], i.e. 1.04 for an omnibus that contains books 1-4 and 5.08 for an omnibus that contains books 5-8.

Edit: Just did a quick search and the thread was Kobo: Separate series index (text) and series_index (float)? with the template I'm using in message #16.

You will need to create a Omnibus column add a checkmark to the use series number template on the third tab of the KoboTouch configuration and enter the following in the template field:

program:
	if field('#omnibus')=='yes' then
		series_start = floor($series_index);
		fraction = fractional_part($series_index);
		if fraction ==# 0 then
			result = series_start
		else
			series_end = round(fraction*100);
			if series_end >=# 0 then
				result = series_start & '-' & series_end
			fi
		fi
	else
		result = $series_index
	fi;
	result

I've attached images of what the configuration page looks like, the Omnibus custom column and a screenshot from my Sage. Basically, once I created the custom column, I did a search for books with either box set or omnibus in the title and marked those as yes in the omnibus column and then updated the metadata on ereader.