Ways to substitute specific words with OpenType

gistfile1.txt

# Goal: substitute each letter in the word "while" with the corresponding alternate.

# Option 0 — does not work, error "many-to-many substitution not available"

lookup whileAttrCalt-0 useExtension {
  ignore sub @AllLetters w' h i l e;
  ignore sub w' h i l e @AllLetters;
  sub w' h' i' l' e' by w.alt2 h.alt2 i.alt2 l.alt2 e.alt2;
} whileAttrCalt-0;

# Option 1 — one-to-one

lookup whileAttrCalt-1 useExtension {
  ignore sub @AllLetters w' h i l e;
  ignore sub w' h i l e @AllLetters;
  sub w' h i l e by w.alt2;
  sub w.alt2 h' i l e by h.alt2;
  sub w.alt2 h.alt2 i' l e by i.alt2;
  sub w.alt2 h.alt2 i.alt2 l' e by l.alt2;
  sub w.alt2 h.alt2 i.alt2 l.alt2 e'  by e.alt2;
} whileAttrCalt-1;

# Option 2 — many-to-many is not supported so we do this weird thing to achieve many-to-many

lookup WHILE_SUBS useExtension {
  sub w by w.alt2;
  sub h by h.alt2;
  sub i by i.alt2;
  sub l by l.alt2;
  sub e by e.alt2;
} WHILE_SUBS;

lookup whileAttrCalt-2 useExtension {
  ignore sub @AllLetters w' h i l e, w' h i l e @AllLetters;
  substitute w' lookup WHILE_SUBS h' lookup WHILE_SUBS i' lookup WHILE_SUBS l' lookup WHILE_SUBS e' lookup WHILE_SUBS;
} whileAttrCalt-2;

# Option 3 — one-to-many, then many-to-one, works quite nicely!

lookup whileAttrCalt-3 useExtension {
  ignore sub @AllLetters w' h i l e, w' h i l e @AllLetters;
  sub w' h i l e by w.alt3 h.alt3 i.alt3 l.alt3;
  sub w.alt3 h.alt3 i.alt3 l.alt3 h' i' l' e' by e.alt3;
} whileAttrCalt-3;

# Option 4 — does not work as desired. Will sub single letters everywhere.

lookup whileAttrCalt-4 useExtension {
  ignore sub @AllLetters w' h i l e, w' h i l e @AllLetters;
  sub [w h i l e]' by [w.alt2 h.alt2 i.alt2 l.alt2 e.alt2];
} whileAttrCalt-4;

# Suggestions? Is there another way?