As leppie says, there is no direct support for partial, although you can achieve a similar effect with an extension of the internal type . F # supports methods internal, so your example would look like this:
type Device() =
inherit MarshalByRefObject()
...
type Device with
member internal this.FindTagName(name:string, tag:OneTag) =
listFuncSect
|> Seq.exists
(fun fs ->
fs.listTags
|> Seq.exists (fun ot -> ot <> tag && ot.name = name))
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