Can you just put a shortcut to Plex in the startup folder so when the htpc reboots, Plex is launched? I haven't tried this, but I don't know of other programs that will let you stream to the web to your devices. Why the developers haven't gotten Plex to work as a service yet is beyond me, but I know it's been req. for quite some time. Just out of curiosity, what are your htpc specs?
Rig: i7 2700k, asrock extreme 4 gen3, 60 gb OCZ Vertex ssd, 128 gb Plextor M3 ssd, samsung spinpoint 1tb hd, 16gb gskill sniper series, EVGA GTX 670 FTW, antec hcg 750w psu
Server: Intel g630, Biostar H61, 4gb gskill ram, antec 400w eco psu