If you don't allow the indentation based parsing to be nested within a bracket-based expression, it doesn't look too bad. At the top level you have the indentation-based parser. When that sees an open bracket, it recurses to the regular parser which doesn't care about indentation.
I'm reasonably sure you could conceal this surface syntax so that macros don't know it exists and work fine. You can call a macro by writing the invocation syntax using the indented format, or by the bracketed format, or a mixture.
It's been done before; see Scheme SRFI-110, a.k.a. Sweet Expressions or t-expressions:
Yes, for a general language (such as abstraction algebra) you would want to allow mixing normal term language and blocks. In RX, that is easy to do: Just reuse the blocks that RX already gives you, while the lines of a block are there for the term language.
