New to PLEX as of a few weeks ago...someone showed it to me and it seemed awesome. So I set it up on an old PC I had at home.
I didn't like the idea of having that computer run all the time. Of course, googling "PLEX + WOL" only comes up with people complaining how the app doesn't have a built in wake on lan function. Not a big help.
My first thing was to just use a separate app to send a Wake on LAN/WAN message to my server. This worked just fine...for me. Then I realized that I'd have to install and configure this same app for other people on their phone if I wanted to share the server with them. This seemed really inconvenient.
So, here's what I did.
Flashed my Linksys E4200 v1 with tomato. dd-wrt or openWRT will work just fine for this. Basically, a router where you can run scripts and inject iptables rules. The router was giving me problems with stock firmware so I was happy to have a reason to flash custom fw.
In Administration > Scripts, added the 2 following lines to the Firewall section
 # Rule to log all new connections on LAN for the plex server for the WOL script to trigger
 iptables -I FORWARD -i br0 -p tcp --dport 32400 -m state --state NEW -j LOG --log-prefix "PLEX LAN Connection "

 # Rule to log all new connections on WAN for the plex server
 iptables -I FORWARD -i vlan2 -p tcp --dport 32400 -m state --state NEW -j LOG --log-prefix "PLEX WAN Connection "
Basically, all new TCP connections to dst port 32400 are sent to /var/log/messages. They look like this:
 Jul 30 14:13:04 unknown user.warn kernel: PLEX WAN Connection IN=vlan2 OUT=br0 SRC=xxx.xxx.xxx.xxx DST=192.168.1.yyy LEN=60 TOS=0x00 PREC=0x00 TTL=49 ID=4796 DF PROTO=TCP SPT=51833 DPT=32400 SEQ=3659947144 ACK=0 WINDOW=65535 RES=0x00 SYN URGP=0 OPT (020403DA0402080A02144E340000000001030308)
(Note, you could just omit the -i in the FORWARD chain and combine them into a single rule, but I have them separate just in case I want to do something different on WAN vs LAN).
Anyway, I have one final script which is checking that /var/log/messages file every few seconds for new lines. If it finds the above line, it tries to ping the plex server. If no response, sends a wol message.
Here is the full script (I put it in the 'WAN Up' section of the event scripts):
 #!/bin/sh

 TARGET=<ip of plex server>
 MAC=<mac of plex server>
 PORTS="(32400)"

 INTERVAL=2
 NUMP=1

 OLD_LC=`wc -l /var/log/messages | awk '{print $1}'`

 while sleep $INTERVAL
 do
     # Only care about new lines since the script last ran
     LC=`wc -l /var/log/messages | awk '{print $1}'`
     NEWLINES=`expr $LC - $OLD_LC`
     if [ "$LC" -ne "$OLD_LC" ]; then
         # Could handle WAN vs LAN different if I wanted. Just do the same thing on either for now...
         LINE=`tail -$NEWLINES /var/log/messages | egrep "PLEX .* DST=$TARGET .* DPT=$PORTS" | tail -1`
         SRC=`echo $LINE | awk '{print $10}' | sed -e "s/SRC=//g"`
         if [ "$SRC" != "" ]; then
             # Found a matching line. Try to ping the server
             RET=`ping -c $NUMP -W 1 $TARGET 2> /dev/null | awk '/packets received/ {print $4}'`
             if [ "$RET" -ne "$NUMP" ]; then
                 # Guess it's sleeping. Send WoL.
                 echo "[`date -Iseconds`] $SRC causes WOL. Line was $LINE"  >> /var/log/wol
                 /usr/sbin/ether-wake $MAC
                 # Could sleep for 20 minutes I guess...I mean, there's no real reason to check again.
                 # Whatever...10 seconds is fine.
                 sleep 10
             fi
         fi
     fi
     OLD_LC=`wc -l /var/log/messages | awk '{print $1}'`
 done
And that's it. I have my server sleep in 20 minutes of no activity...so it is off most of the time. But, whenever someones opens a plex app (android, windows, etc), it will turn on my server (and yes, it stays on as long as stuff is streaming).
It takes about 10-12 seconds for my media to show up from the hibernate state which isn't too bad. Not sure where the bottleneck is...possibly because I don't have a solid state drive for my OS on the server. The script will take a maximum of 3 seconds to send the WoL and a minimum of 1 second (ping on tomato has a minimum of 1 second ping reply wait). The PLEX app might just wait a few seconds for server discovery if it doesn't get a reply right away. Haven't really tried to shorten it yet.
Hope someone here finds this useful!