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People have done and are doing dc measurements: Monitoring power usage on a 24Volt DC battery solar system using Emontx, it's just that most people are interested in monitoring the grid and PV infeeds. If you are limiting yourself to a 'plug and play' solution, then I'm afraid you will have to go down the ac route and monitor your inverter output. But the question I'd ask: Does your inverter give you that information anyway, and can you extract it? If it does have a data output that you can interrogate, then maybe you won't need to do any electronic construction.
Assuming that you've got your data, either by measuring or asking for it, then if you're able and willing to write some software, you can export the data in whatever format you want. We do use JSON for emoncms, but it seems that would bypass syslog, which you're keen to utilise.

Re. your question about shipping data to syslog, one option is to use the new MQTT event module to publish values to MQTT topic(s) of your choice and then setup mqttwarn (https://github.com/jpmens/mqttwarn#syslog) to monitor those topics and forward the values to syslog. mqttwarn can do transformations and filtering as well as notifications to a huge range of other services.

Likewise, mqttwarn can be used to forward data into EmonCMS (https://github.com/jpmens/mqttwarn#emoncms). I am using this with a modified emontx sketch which publishs values to MQTT topics rather than send via RF (http://openenergymonitor.org/emon/node/5922). mqttwarn then transforms the values into payloads suitable for EmonCMS and sends them via HTTP requests. 

As you can probably tell I am very interested in MQTT! 

Hi,

I was wondering the same thing about using logstash and the whole ELK stack. Finally got something working...
I use the logstash input plugin http_poller. Maybe there's a better way. Below is my config. It surely could have improvements but it's my first shot with all these tools. Would be more than happy to have comments.

I'd prefer Emonpi to send its data to logstash as soon as available but didn't find how to yet, so I poll every minute which reduce the resolution compared to the 5s refresh.

input {
  http_poller {
    urls => {
      emonPI_ct1 => "http://192.168.0.89/emoncms/nodes/5/rx/1?id=5&apikey=XXXX"
      emonPI_ct2 => "http://192.168.0.89/emoncms/nodes/5/rx/2?id=5&apikey=XXXX"
      emonPI_T_out => "http://192.168.0.89/emoncms/nodes/5/rx/5?id=5&apikey=XXXX"
      emonPI_Vrms => "http://192.168.0.89/emoncms/nodes/5/rx/4?id=5&apikey=XXXX"
    }
    request_timeout => 30
    interval => 60
    codec => "json"
    # A hash of request metadata info (timing, response headers, etc.) will be sent here
    # metadata_target => "poll_metadata"
  }
}

filter {
  mutate {
    lowercase => [ "name" ]
    convert => { "value" => "float" }
    add_tag => [ "emon" ] 
  }
  if [unit] == "W" {
    mutate { 
      rename => { "value" => "value_%{name}" }
      add_field => { "type" => "power" }
    }
  }
  if [unit] == "C" {
    mutate { 
      rename => { "value" => "value_%{name}" }
      add_field => { "type" => "temperature" }
    }
  }
  if [unit] == "V" {
    mutate { 
      rename => { "value" => "value_vrms" } 
      add_field => { "type" => "tension" }
    }
  }
  if [unit] == "%" {
    mutate {
      rename => { "value" => "value_%{name}" }
      add_field => { "type" => "humidity" }
    }
  }
}

output {
  elasticsearch {
    index => "emonpi-%{+YYYY.MM.dd}"
  }
}

Once in Elasticsearch, I run Kibana for visualization and dashboard.