We describe a technique for molecular-beam-epitaxy-based fabrication, of unstrained quantum dashes with AlxInyGa1-x-yAs alloys lattice matched to InP substrates. Templates for lattice-matched quantum dash growth are obtained by combining molecular beam epitaxy with in situ etching by arsenic bromide. A seed layer of strained self-assembled InAs quantum dashes is converted into nanotrench templates through overgrowth followed by strain-enhanced etching. We have explored limitations on the accessible range of alloy compositions imposed by the etch process and found that strain-induced etching is limited,to compounds with low Al content. Nanotrench templates can be filled with lattice-matched alloys of varied compositions to define barriers and quantum wires that could lead to optoelectronic devices in a spectral range around 1.5 mu m.