Maybe I've misunderstood what you are waiting for but I have installed the SignalK-mqtt-gw plugin through the SignalK appstore.
This instals a local broker and I now have 5 x ESP8266 devices sending and receiving with Openplotter/SignalK using MQTT.
I also send/Receive MQTT through a public broker for remote systems monitoring.
Im no coder but it works great and I'm happy to share more details if requested.
Code:
[{"id":"ebd88b62.b01cd8","type":"ui_gauge","z":"c40476e5.e10638","name":"Start Volts","group":"f0179916.a0d8d8","order":1,"width":"3","height":"2","gtype":"gage","title":"<font size=\"4\">Start Battery Volts","label":"Volts","format":"{{value|number:2}}","min":"10","max":"16","colors":["#ff0000","#00ff00","#ff0000"],"seg1":"12.6","seg2":"14","x":770,"y":80,"wires":[]},{"id":"6fcad3aa.a4578c","type":"ui_gauge","z":"c40476e5.e10638","name":"Start Volts","group":"de1e2f25.504f5","order":1,"width":0,"height":0,"gtype":"gage","title":"<font size=\"4\">Start Battery Volts","label":"Volts","format":"{{value|number:2}}","min":"10","max":"16","colors":["#ff0000","#00ff00","#ff0000"],"seg1":"12.6","seg2":"14","x":770,"y":120,"wires":[]},{"id":"faf2fe8.7751a","type":"mqtt in","z":"c40476e5.e10638","name":"Start Battery voltage ESP(1) ","topic":"electrical.batteries.start.voltage","qos":"0","broker":"f4132a1b.afd788","x":140,"y":80,"wires":[["769b4ccb.209514","fc151291.35ac3","3f0e3d2b.a8e042"]]},{"id":"432d760d.22cea8","type":"mqtt out","z":"c40476e5.e10638","name":"","topic":"Canopus/electrical/batteries/start/voltage","qos":"0","retain":"","broker":"8fcc07d7.a30c58","x":860,"y":200,"wires":[]},{"id":"769b4ccb.209514","type":"delay","z":"c40476e5.e10638","name":"","pauseType":"rate","timeout":"5","timeoutUnits":"seconds","rate":"1","nbRateUnits":"1","rateUnits":"minute","randomFirst":"1","randomLast":"5","randomUnits":"seconds","drop":true,"x":450,"y":160,"wires":[["432d760d.22cea8"]]},{"id":"fc151291.35ac3","type":"trigger","z":"c40476e5.e10638","op1":"","op2":"0","op1type":"pay","op2type":"num","duration":"30","extend":true,"units":"s","reset":"","bytopic":"all","name":"Zero if signal lost","x":470,"y":80,"wires":[["ebd88b62.b01cd8","6fcad3aa.a4578c"]]},{"id":"a1e87d92.4b919","type":"signalk-send-pathvalue","z":"c40476e5.e10638","name":"Send to SK","source":"ESP1","x":770,"y":160,"wires":[]},{"id":"3f0e3d2b.a8e042","type":"function","z":"c40476e5.e10638","name":"Convert to number","func":"str = msg.payload\n//msg.payload = str.substring(5,9);\nmsg.payload = Number(msg.payload);\nreturn msg;","outputs":1,"noerr":0,"x":470,"y":120,"wires":[["a1e87d92.4b919","ebd88b62.b01cd8","6fcad3aa.a4578c"]]},{"id":"f0179916.a0d8d8","type":"ui_group","z":"","name":"DC Volts","tab":"6b55b319.843bbc","order":3,"disp":true,"width":"3"},{"id":"de1e2f25.504f5","type":"ui_group","z":"","name":"Start Battery","tab":"26a5f270.6828ce","order":1,"disp":true,"width":"3"},{"id":"f4132a1b.afd788","type":"mqtt-broker","z":"","name":"","broker":"10.10.10.1","port":"1883","clientid":"","usetls":false,"compatmode":true,"keepalive":"60","cleansession":true,"birthTopic":"","birthQos":"0","birthPayload":"","closeTopic":"","closePayload":"","willTopic":"","willQos":"0","willPayload":""},{"id":"8fcc07d7.a30c58","type":"mqtt-broker","z":"","broker":"test.mosquitto.org","port":"1883","clientid":"","usetls":false,"compatmode":true,"keepalive":"60","cleansession":true,"birthTopic":"","birthQos":"0","birthPayload":"","willTopic":"","willQos":"0","willPayload":""},{"id":"6b55b319.843bbc","type":"ui_tab","z":"","name":"Engine & Tanks","icon":"dashboard","order":1},{"id":"26a5f270.6828ce","type":"ui_tab","z":"","name":"D.C. Power","icon":"dashboard","order":2}]
