Have a look into some of these variables/options below, use google+wiki;
uuid_displace - allows you to inject audio into a channel
temp_hold_music - allows you specify music during transfer hold
campon - places call into parking when on hold (not 100% on this, needs clarification)
You could also modify the MOH file to play the message you require at the desired intervals. Realistically I think this would be the best way, as this will give you much more control over the quality delivered (fading, volume, etc).
Hope this helps
Cal