There's Plenty of research on Langar being served in meat and in some temples it still is:
Page 38 here:
http://en.wikipedia.org/wiki/Langar_(Sikhism)#cite_ref-6Singh, Prithi Pal (2006). "3 Guru Amar Das". The History of Sikh Gurus. New Delhi: Lotus Press. pp. 38. ISBN 81-8382-075-1.
http://books.google.co.uk/books?id=EhGkVkhUuqoC&printsec=frontcover&dq=The+History+of+Sikh+Gurus+By+Prithi+Pal+Singh#v=onepage&q&f=false
Page 241 here
http://en.wikipedia.org/wiki/Langar_(Sikhism)#cite_ref-8S.R. Bakshi, Rashmi Pathak,, ed. (2007). "12". Punjab Through the Ages. 4 (1st ed.). New Delhi: Sarup and Sons. pp. 241. ISBN 81-7625-738-9 (Set).
http://books.google.co.uk/books?id=-dHzlfvHvOsC&pg=PA7&dq=Punjab+Through+the+Ages+By+S.R.+Bakshi,+Rashmi+Pathak,+Rashmi+Pathak+volume+4#v=onepage&q=Punjab Through the Ages By S.R. Bakshi, Rashmi Pathak, Rashmi Pathak volume 4&f=false
A History of the Sikh People by Dr. Gopal Singh, World Sikh University Press, Delhi ISBN 978-81-7023-139-4.
"However, it is strange that nowadays in the Community-Kitchen attached to the Sikh temples, and called the Guru's Kitchen (or, Guru-ka-langar) meat-dishes are not served at all. May be, it is on account of its being, perhaps, expensive, or not easy to keep for long. Or, perhaps the Vaishnava tradition is too strong to be shaken off."
On Variations of Langar:
http://en.wikipedia.org/wiki/Langar_(Sikhism)
Langar (Punjabi: ਲੰਗਰ, Hindi: लंगर) is the term used in the Sikh religion or in Punjab in general for common kitchen/canteen where food is served in a Gurdwara to all the visitors (without distinction of background) for free. At the langar, only vegetarian food is served, to ensure that all people, regardless of their dietary restrictions, can eat as equals. Langar is open to Sikhs and non-Sikhs alike.
The exception to vegetarian langar is when Nihangs (in India) serve meat<sup id="cite_ref-0" class="reference">[1]</sup> on the occasion of Holla Mohalla,<sup id="cite_ref-1" class="reference">[2]</sup> and call it Mahaprasad.<sup id="cite_ref-Mahaprasad_2-0" class="reference">[3]</sup> There are also variations on langar, for example at Hazur Sahib,<sup id="cite_ref-3" class="reference">[4]</sup><sup id="cite_ref-4" class="reference">[5]</sup> where meat is included. Langar is also a common term used across various units in the Indian Army, when referring to a mess<sup class="Template-Fact" style="white-space:nowrap;">[citation needed]</sup>, especially when there is no building and the food is served in open air (or through temporary arrangements like tents).
The reason why meat is no longer served at most langar is, that Langaar is open to all. If we started having meat, the Vaishnav minded Sikhs would start crying and not come. Muslims who attend would say it's not Halal etc etc. Much safer to have a vege option....although even that in some temples is being challenged who don't want garlic etc.
