Hi Tom,
I'm NOT an expert of Lydney or Richard Thomas Co, but it does seems entirely reasonable to think there was a Memorial. However my internet searches this evening haven't helped much ! I don't currently have a subscription to the BNA old newspapers website, but I can still search their database and I cannot find anything at all with references to "Richard Thomas" or "tinplate" as part of my search criteria, and I would have expected the newspapers to carry a mention of sorts, as they did for other Memorials, such as the plaque erected at Cinderford school, for example. However, despite the FoD area being well-served by the Gloster Citizen at that time as the BNA site shows, the BNA site does not hold old copies of the Dean Forest Mercury / Lydney Observer which may perhaps have given the Lydney Memorial more coverage; these records are held on microfiim at Cinderford Library.
http://www.britishnewspaperarchive.co.uk/search/results/1920-01-01/1929-12-31?basicsear...

That said, I also cannot find any BNA site newspaper references to the plaque in St John's Church Cinderford in memory of "The employees of Lightmoor, Speech House and Eastern United Collieries", which was erected by the Crawshay family who owned these mines ?... Perhaps it wasn't thought appropriate for wealthy families such as the Crawshays and maybe Richard Thomas's to "publicise" such memorials ?.

As far as I can glean the Lydney Cross Memorial as per your link was inaugurated c1921, much like most local Memorials were at the time. However it seems there were some ongoing financial problems funding it, or at least the associated new almshouses near it, which the newspapers are reporting for some years afterwards. The papers report that Lord Bledisloe was deeply involved, but again no mention that I've found yet of Richard Thomas Co. Do these point to a lack of involvement from Thomas' ?. It also appears that the Church itself housed a WW1 Memorial, possibly a Stained glass window, and a tablet in the Lych Gate ?. See here for references to this.
http://www.gloucestershire.gov.uk/archives/CHttpHandler.ashx?id=57850&p=0

I suggest contacting St Mary's Church would soon clarify things wrt any Memorial they currently have.
http://www.lydneyparish.org.uk/contacts.php

The Church also appears to have a Stained Glass window dating from WW2, see
http://www.iwm.org.uk_www.iwm.org.uk/memorials/item/memorial/35702

So, if Richard Thomas Co were involved in a Memorial, I wonder if this was limited to the memorial tablet/window at the Church itself, much like the Crawshay one ?.

To find more I suspect we need to search the papers at Cinderford library, and also contact Tata Steel and Glos Archives, who between them hold the Richard Thomas Company records, see
http://discovery.nationalarchives.gov.uk/details/c/F180087

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War Memorial Cross & Almshouses c1930
http://www.sungreen.co.uk/lydney_forest_of_dean/lydney_almshouses.html

For close-up detail see this page, and click on image to enlarge it
http://www.ebay.co.uk/itm/391480952475?clk_rvr_id=1159201650047&rmvSB=true

More history of Lydney Tinplate works and Richard Thomas Co;
http://www.gsia.org.uk/reprints/2003/gi200303.pdf
https://en.wikipedia.org/wiki/Richard_Thomas_and_Baldwins
http://www.gracesguide.co.uk/Richard_Thomas_and_Co
http://wc.rootsweb.ancestry.com/cgi-bin/igm.cgi?op=GET&db=aet-t&id=I10

This prior thread will hopefully interest anyone who's ancestors worked at the tinplate works in Lydney or Lydbrook, I especially recommend the excellent book highlighted in my post of 14th September 2013.
http://forum.forest-of-dean.net/index.php?mode=thread&id=42285