input-observable.ts

import { Input } from '@angular/core';
import { BehaviorSubject } from 'rxjs/BehaviorSubject';

export function InputObservable<T>(inputName?: string) {
    return (target: object, name: string): void => {
        const subject = new BehaviorSubject(this[name]);

        if (delete target[name]) {
            Object.defineProperty(target, name, {
                set(value: T): void {
                    subject.next(value);
                },
                get(): BehaviorSubject<T> {
                    return subject;
                },
            });
        }

        Input(inputName)(target, name);
    };
}

Clever!
I would replace return subject; with return subject.asObservable(); so you don't 'leak' the subject into your class.
Perhaps even pipe in a distictUntillChanged to prevent repeated invocations that might occur.

Nice! unfortunately, it will not work. When you initiate the BehaviorSubject the this context is not what you think it is, it's undefined.