The Ego7 board probably would work in your SL8R, and as the Ego7 and SL8R use the same solenoid, potential issues such as different voltages, power draw, modulation etc. shoulkd be avoided.
However, Planet only warranty their markers for use with the stock board - placing an Ego7 board in the SL8R immediately invalidates any warranty on either the board or the marker - not that the SL8R is likely to still be under warranty, which is one year from the original purchase.
Having said that, Planet are pretty good about honouring the warranty until a year after the marker was considered to have gone out of circulation, which is normally defined as the year the marker was released, and then a year after that (so you can buy a marker at the end of the release year, and still have a years warranty on your new marker).
As such, while I wouldn't promise anything, if you are quick, you might just be able to get the SL8R board replaced under warranty. No harm in contacting Planet and asking.