Hi Matthew.
You cannot pick up values from the excel sheet and use them as filters/prompts for BEx queries. It HAS to be via query binding and as Mark points out below, everything needs to be set to "display as key". Combo boxes, etc will then by default show the short text of the characteristic, i.e. no matter if you have set the default text of the characteristic to be the long text, it will take the short text if the characteristic is set to have it (doesnt matter if short text is empty or not) - so the only way to use the long text is to setup the characteristic to only have long text.
In my case I ended up creating a set of new characteristics only for the dashboard....
Regards
Jacob