attrs.html

<button dynamic="attrs.bind: attrs">Button</button>
<!-- Generated HTML: <button name="foo" type="bar" dynamic="attrs.bind: attrs">Button</button> -->

attrs.ts

...
        this.attrs = JSON.stringify({
            name: 'foo',
            type: 'bar'
        });

dynamic.ts

import {dynamicOptions, autoinject} from 'aurelia-framework';

@dynamicOptions
@autoinject
export class DynamicCustomAttribute {
    constructor(private element: Element) {
    }

    propertyChanged(name: string, value) {
        if (name === 'attrs') {
            let attrs = JSON.parse(value);

            for (let key in attrs) {
                this.element.setAttribute(key, attrs[key]);
            }
        }
    }
}