Fill an IRS W-9 SSN "comb" field so each digit lands in its own printed box — signer types it, not pre-filled

I'm building an embedded signing flow (JWT auth, envelopes created via the eSignature REST API). One of my documents is the IRS Form W-9. The SSN area is a "comb" field — nine small printed boxes (3 + 2 + 4, with a printed hyphen between each group).

My goal: let the signer type their full 9-digit SSN into a single field, and have each digit appear centered in its own printed box on the final signed document.

Important constraint: for privacy, the SSN must be typed by the signer inside DocuSign — I cannot collect it on my server and pre-fill the tab value.

What I've already tried:

• A single Text tab over the SSN row → the number renders in one spot and doesn't line up with the printed boxes.
• The native SSN tab → same single-position rendering, and its dashes overlap the form's own printed dashes.
• Three Text tabs (one per group, 3/2/4) and nine single-character Text tabs, in Courier at a size where each glyph is one cell wide → these do align each digit to its cell, but the signer has to type into multiple separate boxes.
• Formula/calculated fields → these seem to support numeric math only, not string operations, so I can't split one field's value into per-character fields.

My questions:

1. Is there any supported way to have the signer type the SSN once (one field) and have DocuSign distribute each digit into separate positioned cells on the document?
2. If not, is "separate text fields per group/character" the recommended pattern for comb fields, or is there a better approach I'm missing?
3. Is there any way to control character spacing/kerning on a Text or SSN tab so a single-field value could align to a comb?

Thanks!