Cyclone V FPGA to HPS at high frequencies

Sounds about right. If the trigger comes in at a rate of 5MHz then I would use that to capture the samples then make them available through a memory mapped interface connected to the HPS which software can pull the data out.

The other idea closer to yours would be to send the samples into a DMA engine which then writes them into HPS memory. That approach will have less overhead since the hardware does the heavy lifting. There are a couple of DMA engines you could use for this, I recommend using the modular SGDMA in ST-to-MM mode. That DMA engine is available in 14.0 but if you are using older tools you can grab it here: http://www.alterawiki.com/wiki/modular_sgdma You would use that periodic strobe signal to write the 70-bit data into the DMA (use a 128-bit DMA and pad the upper bits) and it would be responsible for writing the 128-bit data into memory. Alternatively you can serialize the data so that you don't have all that wasted memory due to padding but given that 70 bits isn't a nice power of two I don't think it's worth it given all the SDRAM you have connected to the HPS.