User metadata (such as ID and created date) should be available to sync to Marketo

ksenzee, you're correct that these fields are not accessible for field mapping but you can get to them if you have Rules enabled on your site. There are three default rules included in the module which perform the same action as the triggered events which can be turned on/off in the User Integration tab of the module config. If you want to try it, disable the "User Registration / Creation" option on that tab then enable the rule "Update Marketo lead on user creation". Edit the rule then edit the action "Add an item to a list". You'll be able to access [account:uid] [account:name] [account:created] etc. from there and can add them to whatever Marketo keys you'd like.

Example rule export:

{ "rules_update_marketo_lead_on_create" : {
    "LABEL" : "Update Marketo lead on user creation",
    "PLUGIN" : "reaction rule",
    "ACTIVE" : false,
    "OWNER" : "rules",
    "TAGS" : [ "Marketo" ],
    "REQUIRES" : [ "rules", "marketo_ma" ],
    "ON" : { "user_insert" : [] },
    "DO" : [
      { "variable_add" : {
          "USING" : { "type" : "list\u003Ctext\u003E" },
          "PROVIDE" : { "variable_added" : { "marketoparams" : "Marketo Parameters" } }
        }
      },
      { "list_add" : {
          "list" : [ "marketoparams" ],
          "item" : "Email|[account:mail]\r\nMarketoFieldA|[account:uid]\r\nMarketoFieldB|[account:name]"
        }
      },
      { "marketo_ma_rules_action_add_lead" : { "account" : [ "account" ], "data" : [ "marketoparams" ] } }
    ]
  }
}

This may get you what you need but I'll look at adding them directly in the core functionality so that sites without Rules can get to them.

Here's a patch for 7.x-1.x branch that adds the feature.

Feature added to 7.x-1.x

With marketo_ma_user enabled, fields can be mapped on the module config page, User Integration tab.