This posting is older than 6 months and can contain outdated information.
As far as the info provided by Sling is correct, "+" is the default 
setting for channel up - and "-" for channel down.
I fully depend on user feedback regarding the SlingPlayer Behaviour. 
The SlingBox is not available in Germany, so I have no access to one 
and consequently also can't add advanced features like automatically 
recognizing changed key settings.
Regarding custom hardware: this is not feasible in Germany for a small 
company. The government decided to put into place a e-waste 
"recycling" system, that put prohibitive fees in place for small 
companies. The baseline credo: produce small amounts of e-waste, but 
pay for containers full of them (although you didn't produce even 
close as much). That is, plus the same fees like major corporations. 
Of course. Annually. Major corporations that produce off shore without 
paying attention to environmental pollution are hardly affected, while 
smaller companies that DO care can more or less close down right away.
Best regards, 
Felix