Lightroom :: How To Change Caption During Export With A Plugin

I would like to change caption during export with a plugin.

I've got a website with a galery made with piwigo and I use the piwigo's plugin to publish / export my photos to the website.

I'd like to complete my caption with something like that for example:

New Caption =>
{city} - {country} - {dateTime[%B %Y]}
{copyright}
{caption}
 
I've tried different solution without success:

1 - Using LR Transporter plugin It's not the solution because this plugin change caption in LR. I want to change only during export.
2 - Using rc Exportant pluginI don't manage to change something in the preset. I've got a bad level in english and I don't really understand what to do to make what I want
3 - Doing my own plugin I've tried with lightroom SDK. I manage to do a box in export box to complete some metadata. But I'm not a programmer, only a photographer, and I don't know how to do after.
 
I know that it's possible to do something like that. I use jf Flickr plugin and ther is a place to do that in this plugin (Metadata Management / Image Title - Description).

I'd like to do exactly the same with piwigo's publish.