Even though HK rates it from 7.2v to 12v, I've tested it from 4v to 17.3v, which means it will work with a 4 cell. Past 17.3v, the text starts bouncing up and down and the reported voltage drops. But since the peak voltage of a 4 cell is 500mv lower, its not a big deal.
In case you fry your OSD in flight, the Video In
and Video Out
pins are connected, so you'll still get video, just no text. This also means that wiring your plane initially is easier, since you only have to connect video to one pin, not two.
Onto RSSI mods...
EDIT: I just tested this mod with my DragonLink. Works great.
The BAT2 port (not isolated) on the OSD has a voltage range from 0.3v to 17v. Internally, there is a voltage divider (with values 4.7k and 1k) that steps this down so the 3.3v micro can read it. This is nice if you want the voltage of a second battery, but the the Corona receiver has an RSSI range from around 0.7v to 1.4v. This means that you only get two steps of resolution over your entire RSSI range.
By removing the 1k resistor, and therefore disabling the voltage divider, the range decreases from 0-17v to 0-3v, with the tradeoff of much greater accuracy. This fits the Corona RSSI voltage range better. However the OSD will still inflate the voltages, so 1v of input will read a lot higher (~6v) on the OSD
On the PCB, the resistor you need to remove is in red. I recommend desoldering it by putting a solder blob on one of the pins, and then heating the solder blob until both sides melt, then just pushing it off