I did the implementation in our test system based on version 1.0.0a3.

You will find attached the files modified to implement the feature. If the solution is acceptable for you, you can implement it in the feature release. It will be my contribution.

Enjoy it anyway !!!

Note: read the readme because some setting that where done in my custom files should require to be placed in MANTIS standard cinfiguration files.

FEATURE OVERVIEW

All MANTIS usual access level threshold or feature defined with config_defaults_inc.php and config_inc.php are respected, including workflow features and action groups (view_all_bug_page). The only deference is that once an issue is DRAFT, it is considered as now modifiable for everybody except the reporter or at least a manager access level (global or project).

In addition, as long as the issue is not NEW, the reporter or manager can update SOME FIELDS and save again. But they cannot add NOTES (nor update or delete existing notes), send reminders or create childs.
The fields that can be updated are those available form the simple or detailed submission form.

IMPLEMENTATION GUIDE

Once the last modifications are done (either in standard MANTIS files or in your custom files), to activate the feature, you have to define the following in your custom_inc.php:

In : config_inc.php

Activate draft feature by setting the value to DRAFT

$g_bug_draft_status = DRAFT; # instead of '%bug_submit_status%';

#define workflow components if required
$g_status_enum_workflow[DRAFT]= '10:new'; # for instance
$g_set_status_threshold = array( 
    NEW_ => REPORTER,
    FEEDBACK => MANAGER,
    ACKNOWLEDGED => MANAGER,
    CONFIRMED => MANAGER,
    ASSIGNED => MANAGER,
    VALIDATED => REPORTER,
    RESOLVED => DEVELOPER,
    CLOSED => MANAGER );

#finaly you may need/want to tune Email setting to 
    # avoid notifications as long as issue is draft using
    # $g_default_notify_flags, $g_notify_flags

Is this feature completely implemented?

Can I copy the patch to incorporate the DRAFT feature?

Considering that the uploaded code dates back to 2005 and was apparently developped for an alpha version of Mantis 1.0, it is highly unlikely that you will be able to apply it as-is.