I suggested in the discussion area that "veinte" might mean "time", as in "the time is coming/has come", but in the light of another example and a bit more consideration I don't think that's quite right.

I still think it has to do with the use of "veinte" derived from the 20-centavo coin used in the past to make a call from a public phone, most commonly found in the expression "caer el veinte". Here is the reference I posted before, where this expression is explained well:
http://algarabia.com/de-donde-viene/caer-el-veinte/

It's also in the very useful online Diccionario del español de México, of which I've made use before:

"veinte
"3 Caerle el veinte (Coloq) Comprender o recordar algo una persona repentinamente: “Le expliqué durante media hora, hasta que le cayó el veinte”, “A los treinta años nos cayó el veinte de que no nos querían”"
http://dem.colmex.mx/moduls/BuscadorAvanzado.aspx

As I said before, it reminds me irresistibly of "the penny dropped".

In the expression "Mientras nos dure el veinte", which I found in an article in Rolling Stone magazine (where it was explained as an allusion to "el tiempo transcurrido cuando uno ponía una moneda de veinte centavos en los viejos teléfonos públicos"), "veinte" did seem to mean "time". But I think this misled me.

"Venir el veinte" seems, from the few cases available, to be close to "caer el veinte". In other words, "le viene el veinte"/"le ha venido el veinte" seems to means "he/she understands/has understood", or "he/she has got the message". In the reference I quoted before, I now think that's the meaning. It's talking about the PRI in relation to the 2018 presidential election, and saying that some of them are aware of how things really are, but others, including Lastiri, are not. Let me give a bit more context:

"Es obvio que Lastiri tiene una mala lectura del futuro. Se está equivocando. [...]
Y el razonamiento, aunque descabellado, ya lo entendieron Blanca y Doger; por eso quieren el sacrificio, por eso apuestan al martirio y por eso mismo estarían pensando incluso en las mieles de la derrota. A los otros, todavía no les viene el veinte, digamos a Lastiri."
http://www.diariocambio.com.mx/2014/opinion/por-las-entranas...

It now seems obvious to me that "todavía no les viene el veinte" means "they still haven't got the message". This is confirmed by another example, again about the PRI:

"Ellos ya lo entendieron: Zavala, Doger, Amador, Islas y Morales, detrás de un gran líder estatal del PRI hay un gran líder político del estado. Y así tendrían que interpretarlo los potenciales relevos de Blanca: Javier García, Valentín Menéses, Mario Montero, Jorge Estefan, Pablo Fernández, Víctor Chedraui y el resto del pelotón. Pero, a los que no les ha venido el veinte es a los críticos y analistas de la política local que juegan con espejismos de una decisión no de encuestas, modos ni plazos."
http://www.quintacolumna.com.mx/2009/septiembre/columnistas/...

There are many ways you might express it, and I'm not totally sure what kind of expression best reflects the tone, but I'm fairly confident of the meaning.

I had this totally wrong previously, but it subsequently occurred to me that it could be a cartel member's alias. Often these songs have plenty of authentic details, "dog whistles" if you like, which only people with a certain level of knowledge about the scene will understand.

Not sure if we should translate the name (Twenty?), but this appears to refer to one of the now incarcerated lieutenants of the Sinaloa Cartel.

This looks to me like an order: "El 20 is coming, [you'd better] be there/show up".

CIUDAD DE MÉXICO. Jesús Peña, apodado ‘El 20’ y jefe de sicarios del Cártel de Sinaloa fue capturado esta tarde en Colonias de San Miguel, Culiacán.
http://www.elfinanciero.com.mx/sociedad/capturan-a-el-20-lid...

Early Thursday Jesus Peña, alias 'El 20' was arrested in Culiacan, Sinaloa. Peña is designated as the security chief of Ismael "El Mayo" Zambada , a leader of cártel del Pacífico
http://www.borderlandbeat.com/2014/02/reports-of-capture-of-...

http://www.milenio.com/policia/Cae-brazo-armado-Mayo-Zambada...