情報源: coreclr/JapaneseCalendar.cs at master · dotnet/coreclr

設定ファイル化されているわけでは無いので、上Core CLRのソース変更が必要なようですね。

internal static EraInfo[] GetEraInfo()
{
    // See if we need to build it
    if (japaneseEraInfo == null)
    {
        japaneseEraInfo = GetJapaneseEras();
        // See if we have to use the built-in eras
        if (japaneseEraInfo == null)
        {
            // We know about some built-in ranges
            EraInfo[] defaultEraRanges = new EraInfo[4];
            defaultEraRanges[0] = new EraInfo(4, 1989, 1, 8, 1988, 1, GregorianCalendar.MaxYear - 1988,
                                               "\x5e73\x6210", "\x5e73", "H");    // era #4 start year/month/day, yearOffset, minEraYear
            defaultEraRanges[1] = new EraInfo(3, 1926, 12, 25, 1925, 1, 1989 - 1925,
                                               "\x662d\x548c", "\x662d", "S");    // era #3,start year/month/day, yearOffset, minEraYear
            defaultEraRanges[2] = new EraInfo(2, 1912, 7, 30, 1911, 1, 1926 - 1911,
                                               "\x5927\x6b63", "\x5927", "T");    // era #2,start year/month/day, yearOffset, minEraYear
            defaultEraRanges[3] = new EraInfo(1, 1868, 1, 1, 1867, 1, 1912 - 1867,
                                               "\x660e\x6cbb", "\x660e", "M");    // era #1,start year/month/day, yearOffset, minEraYear

            // Remember the ranges we built
            japaneseEraInfo = defaultEraRanges;
        }
    }

    // return the era we found/made
    return japaneseEraInfo;
}

ちょうどこの部分かな。

後はこれ。

https://github.com/dotnet/coreclr/blob/master/src/mscorlib/corefx/System/Globalization/JapaneseCalendar.Windows.cs

  private static EraInfo[] GetJapaneseEras()
  {
      int erasCount = WinRTInterop.Callbacks.GetJapaneseEraCount();
      if (erasCount < 4)
      {
          return null;
      }

      EraInfo[] eras = new EraInfo[erasCount];
      int lastMaxYear = GregorianCalendar.MaxYear;

      for (int i = erasCount; i > 0; i--)
      {
          DateTimeOffset dateOffset;

          string eraName;
          string abbreviatedEraName;

          if (!GetJapaneseEraInfo(i, out dateOffset, out eraName, out abbreviatedEraName))
          {
              return null;
          }

          DateTime dt = new DateTime(dateOffset.Ticks);

          eras[erasCount - i] = new EraInfo(i, dt.Year, dt.Month, dt.Day, dt.Year - 1, 1, lastMaxYear - dt.Year + 1,
                                             eraName, abbreviatedEraName, GetJapaneseEnglishEraName(i));    // era #4 start year/month/day, yearOffset, minEraYear

          lastMaxYear = dt.Year;
      }

      return eras;
  }

Windowsの場合、Windows側がWindows Update等で対応すれば問題無いかも。Unix/Linuxもシステム側の同じような感じ。

https://github.com/dotnet/coreclr/blob/master/src/mscorlib/corefx/System/Globalization/JapaneseCalendar.Unix.cs

private static EraInfo[] GetJapaneseEras()
{
    string[] eraNames;
    if (!CalendarData.EnumCalendarInfo("ja-JP", CalendarId.JAPAN, CalendarDataType.EraNames, out eraNames))
    {
        return null;
    }

    string[] abbrevEnglishEraNames;
    if (!CalendarData.EnumCalendarInfo("en", CalendarId.JAPAN, CalendarDataType.AbbrevEraNames, out abbrevEnglishEraNames))
    {
        return null;
    }

    List eras = new List();
    int lastMaxYear = GregorianCalendar.MaxYear;

    int latestEra = Interop.GlobalizationInterop.GetLatestJapaneseEra();
    for (int i = latestEra; i >= 0; i--)
    {
        DateTime dt;
        if (!GetJapaneseEraStartDate(i, out dt))
        {
            return null;
        }

        if (dt < JapaneseCalendar.calendarMinValue)
        {
            // only populate the Eras that are valid JapaneseCalendar date times
            break;
        }

        eras.Add(new EraInfo(i, dt.Year, dt.Month, dt.Day, dt.Year - 1, 1, lastMaxYear - dt.Year + 1,
            eraNames[i], GetAbbreviatedEraName(eraNames, i), abbrevEnglishEraNames[i]));

        lastMaxYear = dt.Year;
    }

    // remap the Era numbers, now that we know how many there will be
    for (int i = 0; i < eras.Count; i++)
    {
        eras[i].era = eras.Count - i;
    }

    return eras.ToArray();
}