The previous method will work if you can extend Project.js with get_route_options_for_new_doc for your custom field. How to Override js function in erpnext v9
If that route is difficult, an easier option may be to modify your custom_doctype.js and check the previous route in on_load… and set the project field
onload: function (frm) {
var prev_route = frappe.get_prev_route();
if (prev_route[1] === 'Project') {
frm.set_value("project", prev_route[2]);
}
}