Hacer un trabajo de marketing multicanal significa tener una estrategia para cada segmento que cubra todos los puntos de contacto clave con los consumidores, a la vez que integra todos ellos para llegar correctamente a su audiencia.

You ask: Do you think I should keep the term untranslated?
I think you should not, because there is a natural translation in Spanish, which is “mercadotecnia multicanal”, i.e. the use of marketing through a series of different channels (see underneath) to reach the customers, so I see no justification to resort again to a foreign expression which, in this specific case, is not at all needed.

It seems to me more difficult to achieve acceptance amid Spanish marketing experts in the use of “mercadotecna” in the peculiar sector where your translation is likely to be used. The publicity and media sector tend to resort constantly to foreign expressions, English in particular, but not only, just because they sound “good” or “cool” und convey a touch of “trendiness”, so if you believe it is advisable and proper to make good use of your mother tongue here, you have probably lost the battle before it was started. This is unluckily my experience related to anglicisms in the media and publicity sector.

I have translated many texts for the Spanish marketing field and I was seldom allowed to write “mercadotecnia”, to me the natural and logical rendering/choice in Spanish. All my efforts to convince the clients about the convenience of maintaining the natural Spanish translation were like whistling in the wind… all in vain.

But translators do have an obligation (just my opinion!) to make good use of languages and they can occasionally set examples, which might perhaps reach wide target audiences later on. So my recommendation would be to ask your client on how to proceed AFTER having explained them in a couple of polite and accurate words the background of the previous lines.

Term definition:
http://www.wisegeek.com/what-is-cross-channel-marketing.htm
What Is Cross Channel Marketing?
Cross channel marketing is the use of consistent marketing messages and experiences to customers through various marketing channels, such as retail stores, websites, call centers, social media, and mobile devices. To accomplish this, comprehensive customer and prospect data must be available to each channel. Cross channel marketing can increase customer satisfaction and sales, but it can be expensive and challenging to implement

This is from Mexico (they use both terms, “marketing” and “mercadotecnia”):
http://www.icomex-mexico.com/upload/Experiencia_2011/pdf/Pre...
Marketing Multicanal
TV & Radio, Cine, Digital, Direct Marketing, Impresos y Revistas, BTL & Promos.
MERCADOTECNIA MULTICANAL
• Comunicar de manera trascendente (emotiva)
• Utilizando canales adecuados que permitan la interacción y la comunicación integrada:
(impactos sinérgicos en todos los puntos de contacto posibles)
• Reforzando con incentivos, y buscando la lealtad con acciones específicas.
• Se trata del establecimiento de una relación.