Running custom internal applications of Asterisk by Manager

I am looking for a way to have asterisk execute a custom application from the Manager API. I have looked inside the code for app_voicemail.c (this is the module I am interested in) and I have seen how the standard apps were implemented (e.g. VoiceMailMain). I have added my custom application for adding a prerecorded message to one box (using the locking and renumbering of asterisk). I have tested from dialplan and it works.
But it works from the dialplan because there I have the *chan var valid. If I try to do that from the Manager API (not from a call-dialplan) I get an error because there is no channel open. I have tried using the console(autoanswer) as channel and it worked. But I have to say this is no good solution at all. I need some way to use the internals of asterisk (folder locking, renumbering of messages) without beeing tied to a channel. Is there such a way?

Thank you very much.

PS: I am using Asterisk 1.2.5.