Having looked for a similar solution we ended up creating a work around whereby you create a link then add in the btn classes, then the btn type classes:
"style_formats": [
{
"title": "Add button",
"items": [
{
"title": "Create button",
"selector": "a",
"classes": "btn"
}
]
},
{
"title": "Solid button",
"items": [
{
"title": "Primary button",
"selector": "a",
"classes": "btn-primary"
},
{
"title": "Secondary button",
"selector": "a",
"classes": "btn-secondary"
},
{
"title": "Success button",
"selector": "a",
"classes": "btn-success"
},
{
"title": "Danger button",
"selector": "a",
"classes": "btn-danger"
},
{
"title": "Warning button",
"selector": "a",
"classes": "btn-warning"
},
{
"title": "Info button",
"selector": "a",
"classes": "btn-info"
},
{
"title": "Light button",
"selector": "a",
"classes": "btn-light"
},
{
"title": "Dark button",
"selector": "a",
"classes": "btn-dark"
},
{
"title": "Link button",
"selector": "a",
"classes": "btn-link"
}
]
},
{
"title": "Outline button",
"items": [
{
"title": "Primary button",
"selector": "a",
"classes": "btn-outline-primary"
},
{
"title": "Secondary button",
"selector": "a",
"classes": "btn-outline-secondary"
},
{
"title": "Success button",
"selector": "a",
"classes": "btn-outline-success"
},
{
"title": "Danger button",
"selector": "a",
"classes": "btn-outline-danger"
},
{
"title": "Warning button",
"selector": "a",
"classes": "btn-outline-warning"
},
{
"title": "Info button",
"selector": "a",
"classes": "btn-outline-info"
},
{
"title": "Light button",
"selector": "a",
"classes": "btn-outline-light"
},
{
"title": "Dark button",
"selector": "a",
"classes": "btn-outline-dark"
}
]
},
{
"title": "Button size",
"items": [
{
"title": "Button small",
"selector": "a",
"classes": "btn-sm"
},
{
"title": "Button default",
"selector": "a",
"classes": "btn-norm"
},
{
"title": "Button large",
"selector": "a",
"classes": "btn-lg"
},
{
"title": "Button block",
"selector": "a",
"classes": "btn-block"
}
}
]
Not ideal but works for now