Simple patch for wscons keyboard in NetBSD. After patching and building mlterm
(cd mlterm-3.8.8; patch -p1 < mlterm-wscons-keyboard.diff) all you need is a
valid keyboard map for wscons (see /etc/wscons.conf file).

Sample definitinion:
keycode 4 = a A aogonek Aogonek
keycode 8 = e E eogonek Eogonek

where 'aogonek' etc. are defined in /usr/include/dev/wscons/wsksymdef.h

--- mlterm-3.8.8/uitoolkit/fb/ui_window.c.orig	2019-03-31 15:10:47.000000000 +0200
+++ mlterm-3.8.8/uitoolkit/fb/ui_window.c	2019-05-02 13:30:44.160503028 +0200
@@ -1898,7 +1898,7 @@
 
 size_t ui_window_get_str(ui_window_t *win, u_char *seq, size_t seq_len, ef_parser_t **parser,
                          KeySym *keysym, XKeyEvent *event) {
-  u_char ch;
+  u_int ch;
 
   if (seq_len == 0) {
     return 0;
@@ -1933,7 +1933,7 @@
   }
 #endif
 
-  if ((*keysym = event->ksym) >= 0x100) {
+  if ((*keysym = event->ksym) >= 0x7f) {
     switch (*keysym) {
       case XK_KP_Multiply:
         ch = '*';
@@ -1950,7 +1950,7 @@
       case XK_KP_Divide:
         ch = '/';
         break;
-
+      
       default:
         if (win->disp->display->lock_state & NLKED) {
           switch (*keysym) {
@@ -1993,6 +1993,25 @@
 
           *keysym = ch;
         } else {
+          /* taken from FreeBSD: sys/kern/subr_terminal.c: 
+           * http://fxr.watson.org/fxr/source/kern/subr_terminal.c?v=FREEBSD-12-STABLE#L283
+           * algorithm was created for UTF-32, but below 0x10000
+           * UTF-16 and UTF-32 are the same
+           * 
+           * surrogate pairs - 0xd800 to 0xdfff aren't supported
+           * there is no printable keys in NetBSD over 0xdfff at this moment
+           */
+          if (ch < 0x800) {
+            seq[0] = 0xc0 | (ch >> 6);
+            seq[1] = 0x80 | (ch & 0x3f);
+            return 2;
+          } else if (ch < 0xd800) {
+            seq[0] = 0xe0 | (ch >> 12);
+            seq[1] = 0x80 | ((ch >> 6) & 0x3f);
+            seq[2] = 0x80 | (ch & 0x3f);
+            return 3;
+          };
+
           return 0;
         }
     }