Well... one step further. The regex fix worked on the 404 but the checksum still fails.
root@Tower:~# /usr/local/emhttp/plugins/Embyserver/scripts/rc.Embyserver install
Installing Emby Server...
--2015-09-26 23:06:33-- https://github.com/MediaBrowser/Emby.Releases/raw/master/Server/MediaBrowser-3.0.5724.5.zip
Resolving github.com (github.com)... 192.30.252.129
Connecting to github.com (github.com)|192.30.252.129|:443... connected.
HTTP request sent, awaiting response... 302 Found
Location: https://raw.githubusercontent.com/MediaBrowser/Emby.Releases/master/Server/MediaBrowser-3.0.5724.5.zip [following]
--2015-09-26 23:06:34-- https://raw.githubusercontent.com/MediaBrowser/Emby.Releases/master/Server/MediaBrowser-3.0.5724.5.zip
Resolving raw.githubusercontent.com (raw.githubusercontent.com)... 23.235.46.133
Connecting to raw.githubusercontent.com (raw.githubusercontent.com)|23.235.46.133|:443... connected.
WARNING: certificate common name `www.github.com' doesn't match requested host name `raw.githubusercontent.com'.
HTTP request sent, awaiting response... 200 OK
Length: 17575087 (17M) [application/octet-stream]
Saving to: `/boot/config/plugins/Embyserver/Embyserver-PhAzE-3.0.5724.5-src.zip'
100%[====================================================================================================================>] 17,575,087 6.71M/s in 2.5s
2015-09-26 23:06:37 (6.71 MB/s) - `/boot/config/plugins/Embyserver/Embyserver-PhAzE-3.0.5724.5-src.zip' saved [17575087/17575087]
Install failed: Failed MD5 checksum