Hi Geoff,
Instead of an "official" Splunk target you could use the NLog network target https://github.com/NLog/NLog/wiki/Network-target to write to a UDP port on your splunk server.
I've written a guide on using the log4net UDP adapter https://kzhendev.wordpress.com/2015/06/19/send-logs-to-splunk-with-log4net-udpappender/ which you could follow to setup NLog.
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