PicoDVI interlace generation

Question

I would like to generate 480i mode using PicoDVI library. (For vintage hardware emulation purpose)

I've already found out, that 640x480@60 Interlace mode is below minimum DVI bandwidth (25MHz), but I could use pixel-doubling to make the raster lines artificially longer.

I have successfully constructed 1440x240@60Hz mode:

{
    .h_sync_polarity = false,
    .h_front_porch = 40,
    .h_sync_width = 136,
    .h_back_porch = 176,
    .h_active_pixels = 1440,

    .v_sync_polarity = true,
    .v_front_porch = 3,
    .v_sync_width = 10,
    .v_back_porch = 3,
    .v_active_lines = 240,

    .bit_clk_khz = 270000,
}

Which displays on my monitor just fine (as a long, vertical stripe).

What puzzles me is how to convert it to 480i mode. What I understand is I need to trigger a VSync in the middle of the last line of the 1st field, and it will transition into half-line shifted 2nd field. This is analog theory, but how do I do that using DVI signalling?

Is it even possible? DVI specification does not even mention interlace one time.