Assigns Right Alt of HHKB to toggle the input mode (eisuu/kana). Karabiner required. Put the file below in ~/.config/karabiner/assets/complex_modifications .

hhkb.json

{
    "title": "HHKB sei0o settings",
    "rules": [
        {
            "description": "Set Right Alt key to toggle the input mode",
            "manipulators": [
                {
                    "type": "basic",
                    "description": "hogehoge",
                    "from": { "key_code": "right_alt" },
                    "to": [ { "key_code": "lang1" } ],
                    "conditions": [
                        {
                            "type": "input_source_if",
                            "input_sources": [ { "language": "en"} ]
                        }
                    ]
                },
                {
                    "type": "basic",
                    "description": "hogehoge",
                    "from": { "key_code": "right_alt" },
                    "to": [ { "key_code": "lang2" } ],
                    "conditions": [
                        {
                            "type": "input_source_if",
                            "input_sources": [ { "language": "ja"} ]
                        }
                    ]
                }
            ]
        }
    ]
}